Well Craig, since nobody has offered a superficial review of the main amp classes, I'll try it. Class A operation makes all the power output transistors carry a heavy current flow at all times, regardless of whether there's any sound being reproduced at that instant. In pure class B operation, the transistors only carry current at the instant that they're actually amplifying a part of a sound wave. One that amplifies during the positive part of a wave is shut completely off during the negative part and the other transistor(or multiple transistors)is turned on for amplifying the negative part. The net result of these schemes is that the class A is very inefficient(maybe 20%), using power continuously and runs very hot, but since the current is always "on", there's no problem in turning on when the sound wave changes polarity from positive to negative and vice-versa. The class B is a lot more efficient(maybe 60%)and runs cooler, but it has to turn on and off very quickly(all this is happening in millionths of a second)when the sound waves crossover from one polarity to another, and since it can't do it instantaneously from a dead start(i.e. no current flowing), there's some "crossover distortion" introduced at those points. The class AB(somewhat misnamed; mostly B with a little bit of A operation)allows just a little bit of current to flow at all times(but much less than in class A)so that it accelerates from idle rather than a dead start and the crossover distortion problem is reduced while keeping most of the power use efficiency and cooler running of pure class B operation.
As a practical matter, any of the preceding classes can be designed with inaudibly low distortion, so this is more about bragging rights than real-world sound quality and you can pretty much feel free to ignore amp classes. Numbers count, not letters.
