What are the different classes of amps and what does that mean?
The most common types of amps that you will see in car audio are AB, D, and T. These are just ways of defining the method the amp uses in amplifing the signal. AB is a traditional analog method, while D and T are digital based amps. Class D amps are only useable for subwoofers. For more details and specifics, visit this section of the Basic Car Audio site: http://www.bcae1.com/ampclass.htm

We said that class 'A' amplifiers were VERY inefficient. Class 'AB' amplifiers are also inefficient but are more more efficient than class 'A' amplifiers. Class 'AB' mobile amplifiers are generally 60% efficient when driving a 4 ohm load at maximum power (just before clipping). The reason that these amplifier configurations are inefficient is because there is a difference of potential (voltage) across the output transistors and current flowing through the output transistors. When you have voltage across the device and current flow through the device, there will be power dissipation in the form of heat. The power needed to produce this heat is wasted power. When there is (virtually) no voltage drop across a device (such as a large piece of wire or a transistor), there can be a significant amount of CURRENT flow through the device with (virtually) no power dissipation. This means that there is virtually no heat given off (highly efficient). The inverse is also true. If you have a significant amount of VOLTAGE across the device (transistor, wire...) but no current flow through the device, again, there will be no wasted power.

OK, now to the point. A class 'D' amplifier, which may also be known as a switching amplifier or a digital amplifier, utilizes output transistors which are either completely turned on or completely turned off (they're operating in switch mode). This means that when the transistors are conducting (switched on) there is virtually no voltage across the transistor and when there is a significant voltage across the transistor (switched off), there is no current flowing through the transistor. This is very similar to the operation of a switching power supply which is very efficient.