Slew Rate:

This is a term used to describe how quickly the output of an amplifier can track its input. Slew Rate is usually measured in V / usec. The higher the value (up to a point), the better the amp is at potentially reproducing the subtle nuances and dynamics associated with music reproduction.

Speaker Sensitivity and Efficiency.

Speaker sensitivity is a specification provided by all manufacturers of high-quality speakers. The sensitivity rating has no relation to sound quality, as some of the very best speakers have low ratings. Sensitivity ratings simply tell you how much sound a speaker will produce for a given power input.

Sensitivity ratings are given in decibels per watt at one meter, or db/Wm. So, with an input of one watt (usually white noise), a speaker with a sensitivity of 90 db/Wm will produce 90 decibels of sound at a distance of one meter. A sensitivity of 90 is considered average, with ratings of 87 and below considered low sensitivity and above 93 considered high sensitivity. To increase the volume by 3 db, you must double the power. So, using the example above, to make 93 db you would need two watts, and to make 96 decibels, four watts.

Most of the time your system is cruising along producing only a few watts. You need extra power for loud bass passages, crescendos in classical music, and other highly dynamic passages. Your speakers may need more than 10 times the average power to re-create these dynamic passages accurately, and if you are playing loudly to begin with, you may need an awful lot of power if you have speakers with a low sensitivity rating.

So, when you are buying an amplifier, consider your speakers, your vehicle size and how loudly you want to play. If you have sensitive speakers, you probably will not need as much power -- even 20 clean watts would probably be enough. If your speakers are only moderately sensitive, your vehicle is large or exceptionally noisy at highway speeds and you want to play loudly, you will need more power in order to faithfully reproduce dynamic passages.

"Sensitivity," which is expressed in dB, should not be confused with "efficiency" that is expressed as a percentage of power out relative to power in. Efficiency data for loudspeakers suffers from many problems such as failure to consider variations in frequency response.

Speaker efficiency is the ability of the speaker to do work or use power. The more efficient the speaker; the less power is required for the speaker to produce sound. Voice coil design, type and size of the magnets, speaker cone design and material, speaker size, etc. all play a critical role in determining speaker efficiency. However, speaker size is a good general method for guessing efficiency.

Typical speaker efficiency (for physicists) is about 5%. Meaning that for 100% power input, you get about 5% acoustical work back.

Keep in mind that when considering subwoofers, or any speaker that will get more than ~100 watts RMS of power, these measurements are affected by other factors that make this specification less than useful when choosing between speakers.

THD or Total Harmonic Distortion.

Back in the old days (1982) It was FTC mandated for the manufacturer to provide a comprehensive single criteria power specification. However, with the de-regulation craze of the 80's, this requirement was dropped. This left it up to manufacturers to determine how to advertise and display their product specifications with no commonly accepted standard for emasurement.
I'll attempt to explain how THD is measured.

Of a signal, the ratio of (a) the sum of the powers of all harmonic frequencies above the fundamental frequency to (b) the power of the fundamental frequency.

The THD is usually expressed in percent as distortion factor or in dB as distortion attenuation.

Measurements for calculating the THD are made at the output of a device under specified conditions.

Now, there are several ways to measure THD that are commonly used. One is defined by the FCC, and another by the EIA.

Years ago a number of papers were written on human hearing and harmonic distortion. What they found was that the human ear is very insensitive to harmonic distortion that is close to the main signal, and increases in sensitivity to harmonic distortion further away from the main signal. The second harmonic, which is an octave away from the main signal, is the hardest to hear, especially when you are driving a loudspeaker.
The best estimates that I can give you is that we can detect somewhere between 1% and 3% of second order harmonic distortion. Which is why you can't hear it. If the sum total distortions were farther away from the main signal you would be able to hear it. Some solid state designs can have pretty low distortion but they can get to be aggravating after awhile. That's because the distortion generated by the amp is further away from the main signal where the ear is more sensitive.

Without going into too much detail, there are many factors in how THD can be measured, including but not limited to:
is the signal being used to measure THD a notch frequency, of full 20Hz-20KHz at equal power?
Over what unit period of time is THD being measured?
What kind of signal is being used to measure the distortion?
Point being, very few manufacturers specify this data, so THD is helpful at times, but again, not something to base a purchase on.
Remember, some of the best amplifiers in the world have an advertised THD of between 1% to 10%.
It's commonly agreed that distortion below 1% is inaudible, and in a car, below 10% is inaudible to the human ear.

There are two widely accepted ways of measuring THD. One is mandated by the FCC, and is the best way to measure distortion for car audio amplifiers. The other method is defined by the EIA and is far less acceptable for accurate audio amplifier comparisons.

S/N or Signal to Noise ratio

reference links:
http://en.wikipedia.org/wiki/Signal-to-noise_ratio

signal-to-noise ratio (SNR):
The ratio of the amplitude of the desired signal to the amplitude of noise signals at a given point in time.

SNR is expressed as 20 times the logarithm of the amplitude ratio, or 10 times the logarithm of the power ratio.

SNR is usually expressed in decibels (dB) and in terms of peak values for impulse noise and root-mean-square (RMS) values for random noise. In defining or specifying the SNR, both the signal and noise should be characterized, e.g., peak-signal-to-peak-noise ratio, in order to avoid ambiguity.

The sSNR of car amplifiers today is below the threshold of human hearing, so this emasurement is of little use when comparing amplifiers. Factors such as slew rate, damping factor, and power supply voltage are more important in determining the quality of an amp.

Amplifier Damping Factor

Damping factor is rarely published with low to medium grade amplifiers but it is almost always published with high end American amplifiers. And even when it is published, it is rarely published correctly. The damping factor is the ratio between the load impedance and the amplifier's internal impedance (load impedance divided by internal impedance). Like output power ratings, the damping factor is an amplifier characteristic that cannot be represented by a simple number.

An impedance value is a complex number made up of a real term and an imaginary term. The real term comes from the resistance of the object being measured. For example, if you measure the resistance of an 8 ohm driver with an ohmmeter, you will find that it is around 6.3 ohms. Some times, this is referred to as the DC resistance, but this is being redundant because a resistance value by its nature must be taken at DC. The imaginary term is from the inductance and reactance of the object being measured. A driver's voice coil, for example, is made up of winds of wire. The resulting effect is an inductor contributing a significant amount of inductance to the impedance value. There is also a bit of reactance caused by inherent capacitance between parallel wires in the driver assembly but it is usually small enough in a driver that its value is negligible.

Those familiar with a complex value will know that its behavior is dependent on the frequency of the source signal. The impedance of a driver might be 8 ohms at say 100Hz but it could be 30 ohms at 1kHz. Thus any measurement taken that is dependent on the complex impedance of a driver will also be dependent on the frequency of the source signal. So the damping factor of an amplifier will be dependent on the frequency of the signal that the amplifier is generating, which is the reason why you can't just give a single number as the damping factor like most manufactures do.

As if that isn't complicated enough, keep in mind that the impedance graph is different for each driver, the amplifier's internal impedance is also complex, and you have to figure in the impedance contributed by the wires and connectors used in the signal path. In other words, it is impossible to accurately specify a damping factor. This is all fine, but what bothers me is that most high end amplifier manufacturers just publish a number with no indication to the uselessness of such a simple representation of what is a complicated relationship between the amplifier and the load.

Not all amplifier manufacturers are lazy so some come up with ways to specify the damping factor. One way manufacturers specify it is to limit the various conditions that the damping factor is dependent on. Thus they may specify a damping factor of "200 at 1kHz with a 4ohm impedance load at the amplifier output terminals". So in other words, if you put a load with an impedance of 4 ohms at 1kHz across the amplifier's output terminals and the frequency generated by the amplifier is 1kHz, the ratio between the load impedance and the amplifier's internal impedance is 200. Which means that the amplifier's internal impedance at 1kHz is 0.02ohms. The amplifier's internal impedance at 1kHz will stay the same but damping factor will fluctuate depending on the load impedance used and the wires/connectors used in the signal path. For example, if you use a driver with an impedance of 8 ohm at 1kHz instead, then the damping factor becomes 400. Conversely, if you use a driver with an impedance of 2 ohm at 1kHz, the damping factor will be 100.

For reasons that I will indicate later, the damping factor is mainly significant for amplifiers used to power sub woofers. Given that, a damping factor given at 1kHz is pretty much meaningless since sub woofers are usually limited to producing frequencies below 100Hz. How do you get around this then? Well some manufacturers publish damping factors as "greater than 200". What this means is that provided a load with a constant impedance of 4ohms across the frequency spectrum, the damping factor measured at the amplifier's output terminal is greater than 200. Which is the same as saying that the internal impedance of the amplifier will never rise above 0.02ohms from 20Hz to 20kHz. This is the best solution to the problem that I've seen so far since it specifies everything the amplifier's manufacturer can specify. The damping factor specified this way is only dependent on variables controlled by the consumer such as the driver, wire and connectors used. Usually, a damping factor of greater than 50 is considered adequate, though most high end amplifiers have a damping factor of greater than 200.

With that said, why should you care about the damping factor at all? If it is so complicated to specify, why would we want to know it in the first place? Well, the significance of the damping factor is twofold. First, and perhaps more obscure and lesser well known, the damping factor indicates the efficiency of the output device (transistors) used in the amplifier. Second, the damping factor indicates the amplifier's ability to control the motion of a driver.

The load and the output device of an amplifier makes a complete circuit and whatever current flows through the load also flows through the output device. Thus if the amplifier is putting out 2 amperes of current, then the same 2 ampere of current is flowing through the load and the output device of the amplifier. The total power dissipated in the complete circuit is then the current squared multiplied by the total impedance in the circuit. Lets assume a damping factor of 200 for a load impedance of 4ohms. Thus the amplifier's internal impedance is 0.02ohms. The total impedance in the circuit is then 4.02 ohms. Multiplying 2 squared and 4.02 together we get 16.08 watts. Of this power, 0.08 watts is dissipated by the amplifier's output device and the rest is delivered to the load. Thus about 0.5 percent of the power is wasted by the amplifier's output device. Because this percentage of wasted power is rather small compared to the overall wasted power in the whole amplifier (around 50 percent), it is rarely mentioned. But it is nonetheless indicated by the amplifier's damping factor.

The damping factor is most often used as an indication of the ability of an amplifier to control the motion of a driver. When a signal sent to a driver is suddenly stopped, the driver's cone continues moving back a forth for a short period after the signal has stopped. A driver with a cone that stops quickly is said to have a good transient response while a driver with a cone that does not stop quickly is said to have a bad transient response and thus is described as inaccurate.
I think it goes without saying that most people would prefer a driver with good transients and thus would prefer that the cone of the driver stop quickly after the source signal stops.
With tweeters and mid-bass drivers, this not a hard task since the cones of these types of drivers are relatively light and a relatively large motor structure can be used to control the motion of the cone. However, the cone of a low frequency driver is quite sizable and it is physically impractical to use a motor assembly large enough to obtain transient responses as good as that of a tweeter or a mid-bass driver. Thus low frequency drivers usually have relatively poor transient responses.
This is really not too much of a problem since humans are less sensitive to distortion in the low frequencies. In fact, THD of 3 to 6 percent from a low frequency driver is considered acceptable. Low frequency distortion only becomes objectionable when it gets close to 10 percent.
Since drivers are just electric motors, they become generators when their terminals are shorted. If you place an ammeter across the leads of a driver and push the cone up and down, you will see a current flowing through the ammeter. The higher that current is, the more difficulr the cone becomes to move. Thus, if a driver's cone is moving, the quickest way to stop it is to place a dead short across its leads.
The internal impedance of an amplifier is usually very small and in the absence of a source signal, it is like a short across the leads of the driver. The amplifiers with a higher damping factor will have a lower internal impedance so it will be closer to a short, thus the amplifier with a higher damping factor will cause the driver to stop quicker than the amplifier with a lower damping factor. Since low frequency drivers need all the help they can get to stop their cone from moving when the source signal stops, a high damping factor is desirable for an amplifier intended to power low frequency drivers. The damping factor is not as relevant when the amplifier is used to power mid-bass and tweeter drivers since those drivers already have pretty good transient response due to their relatively small cone size.

Amplifier Classes

There are five main amplifier designs: Class A, A/B, B, D, and Tube amplifiers. All of these but tube amplifiers are considered "solid-state."

Class A amplifiers are the most sonically accurate. On the other hand, they have some drawbacks that make them not be the most common choice. Class A amplifiers use only one output transistor that is turned "on" all the time, giving out tremendous amounts of heat. Class A amplifiers are very inefficient (~25%). More heat means more heatsink area, so even though most class A amps have built-in cooling fans, they are big. Pure class A amplifiers are usually expensive.

Class B amplifiers are the most common and use two output transistors. One for the positive part of the cycle and one for the negative part of the cycle. Both signals are then "combined". The problem with this design is that at the point when one transistor stops amplifying and the other one kicks in (zero volt line), there is always a small distortion on the signal, called "crossover distortion". Good amplifier designs make this crossover distortion very minimal. Since each transistor is "on" only half of the time, then the amplifier does not get as hot as a class A, yielding to a smaller size and better efficiency (~50%).

Class A/B amplifiers are a combination of the two types described above. At lower volumes, the amplifier works in class A mode. At higher volumes, the amplifier switches to class B operation.

The class D amplifier (known as digital amplifier) is the last of the solid-state types. These amplifiers are not really digital (there is no such thing), but operate similarly in manner to a digital-to-analog converter (DAC). The signal that comes in is sampled a high rates, and then reconstructed at higher power. This type of amplifier produces almost no heat and is very small in size. Efficiency is much higher in class D amplifiers (~80%).
The sound quality of a Class-D amplifier is much lower than that of other solid-state amplifiers, which is why Class-D amplifiers are only used for subwoofers in car audio. This is because the switching speed of the transistors, and lower sound quality are masked by the lower frequencies being reproduced by the subs, since distortion is harder to discern at low frequency.

other variations on a theme:

Class T: Class T (Tripath) is similar to class D with these exceptions: This class does not use analog feed back like its class D cousin. The feedback is digital and is taken ahead of the output filter, avoiding the phase shift of this filter. Because class D or T amplifier distortion arises from timing errors, the class T amplifier feeds back timing information. The other distinction is that this amplifier uses a digital signal processor to convert the analog input to a PWM signal and process the feedback information. The processor looks at the feedback information and makes timing adjustments. Because the feedback loop does not include the output filter, the class T amplifier is inherently more stable and can operate over the full audio band. Most listeners can not hear the difference between class T and good class AB designs. Both class D and T designs share one problem: they consume extra power at idle. Because the high frequency waveform is present at all times, even when there is no audio present, the amplifiers generate some residual heat. Some of these amplifiers actually turn off in the absence of music, and can be annoying if there is too much delay turning back on.

Class G: Class G improves efficiency in another way: an ordinary class AB amplifier is driven by a multi-rail power supply. A 500 watt amplifier might have three positive rails and three negative rails. The rail voltages might be 70 volts, 50 volts, and 25 volts. As the output of the amplifier moves close to 25 volts, the supply is switched the 50 volt rail. As the output moves close to the 50 volt rail, the supply is switched to the 70 volt rail. These designs are sometimes called "Rail Switchers". This design improves efficiency by reducing the "wasted" voltage on the output transistors. This voltage is the difference between the positive (red) supply and the audio output (blue). Class G can be as efficient as class D or T. While a class G design is more complex, it is based on a class AB amplifier and can have the same clean characteristics as well.

Class H: Class H is similar to class G, except the rail voltage is modulated by the input signal. The power supply rail is always just a bit higher than the output signal, keeping the voltage across the transistors small and the output transistors cool. The modulating power supply rail voltage is created by similar circuitry that you would find in a class D amplifier. In terms of complexity, this type of amplifier could be thought of as a class D amplifier driving a class AB amplifier and is therefore fairly complex.

Lastly we have tube amplifiers, which aren't often used in car audio. Tube amplifiers have about 50 to 60% efficiency.
Tube amplifiers are said to sound more musical. The reason is that tube amplifiers produce even ordered harmonics. Musical instruments give off harmonics in even orders. Transistor amplifiers tend to give off harmonics that are odd ordered. These harmonics are not pleasing to the ear as second order harmonics are. Modern solid state amplifiers have very low distortions but their distortions are less tolerated by the ear than even ordered harmonics. This means that when you hear someone say a Tube amp is "warm" sounding, they are actually talking about the second order distortion produced by that tube amplifier, which they find pleasing to the ear. A good example of this is in guitar amplifiers, which often pride themselves on their second order harmonics.
One should note that while most solid state amplifiers have very low distortions (Total Harmonic Distortion) for the left and right channel, other channels are often much higher as these specifications are rarely noted. Subwoofer amplifiers are particularly bad at creating odd ordered harmonics.
I believe that the best tube and solid state amplifiers sound amazingly alike. Bad tube amplifiers sound tubby and slow. Bad transistor amplifiers sound harsh, bright and strident.

Just like you can't judge a good book by its cover, you can learn very little about an amplifier without digging in and seeing what is inside. Generally speaking, the most important component of any amplifier is its power supply. Is it sufficient? Is it accurate? Is it fast? Unfortunately, almost no amplifier company talks about their power supplies or what transformers they use (An example of a good company would be Eclipse, who uses dual toriodal transformers in their amplifier power supplies.)
I think most manufacturers would prefer you not ask.

I hope this clears up some of the more frequent questions regarding amplifier classes, as well as tube versus solid state amplifiers.