Class A- best fidelity. One transistor handles the entire waveform both positive and negative sides. The only limitation on its fidelity is the linearity of the transistors in the output stages of the amp. The transistors are always partially on so they have to get rid of a ton of heat and as a result are only about 25% efficient.
Class B- two transistors in a push-pull configuration recreate the wave form. One handles the positive side and one handles the negative side. If one is on the other is off, and there is usually audible distortion at the zero point as one switches off and the other on.
Class A/B- Two transistors in a push-pull configuration but with some overlap around the zero point. The positive transistor stays on slightly below the zero voltage line and the negative transistor switches on just above the zero-line. The result is better fidelity than a Class B and better efficiency than a Class A. This is the most common type car amp.
Class C- doesn't even create the whole waveform. Useless for accurate audio reproduction but very efficient. Typiclally used in megaphones and such where battery life is more important than fidelity.
Class D- approximates the wave form by pulsing the rail voltage. The amp is basically a switching power supply with the goal an approximation of the input signal rather than a steady voltage. The output is then sent through a low pass filter to filter out the switching frequency noise. The output devices are high speed devices that are either completely on or completely off. As such there is minimal heat generated by the output devices. In addition to being an approximation of the waveform, the frequency range of the amp is very limited. In order to maximize the efficiency, the switching frequency must be fairly low and a first order filter on the output will leave some artifacts of the switching noise at higher frequencies. Because efficiency is more important than absolute fidelity when it comes to subs, Class D amps are a good choice for subs. Mids and highs don't need as much power and efficiency isn't as important as fidelity so they are not generally made as full range amps, although it can be done.
Class T- uses a proprietary chip to create what is effectively a Class D amp with a variable switching frequency capable of full range operation. It is not a linear amp though it comes pretty close. It does sound different than a Class A or A/B, but most speakers cannot bring out the differences and most people wouldn't know what they are hearing different. They are more efficient than Class A and A/B and not as efficient as Class D. Best case the Class T amp is about 80% efficient.
IMHO Class T amps are not worth the sacrifice in fidelity for the boost in efficiency because typically the difference in current draw real world is on the order of only a few amps. With a sub amp the difference is typically much larger and unless absolute fidelity is your end goal, then a Class D amp is the way to go for subs. Typically you are looking at halving your power consumption on your highest drawing item when going with a Class D over a Class A/B for the same amount of power. For many people, that can easily be the difference between being able to use the stock alternator and having to upgrade.
