Explain Class A amp to non audiophile friends

http://sound.westhost.com/class-a.htm

A reasonable analogy could be:
The transistor is like a lit path. The music is like shadows moving down this path.
If the path is fully lit 100% of the time, then the only issue is the shadows moving, (music)
in an AB amp, the light AND the moving shadows are being constantly varied. So the shadows may be disturbed by the light (Transistor bias) flickering.

First you have to think about the waveform of a sound, it has a peak and a trough and it cycles from peak to trough over and over at a certain frequency.

With a Class A, the amplifing device is drawing more than enough current at all times throughout the whole cyle and is being called on to amplify the whole cycle from peak through trough. There can be one or more amplifying devices -- a tube, a transistor, etc.

With a Class B there has to be at least two amplifying devices per channel -- a pair of transistors or tubes (or two pair or three pair etc).

The signal is split going into the output devices so that one device is amplifying the peak of the waveform and one is amplifying the trough of the waveform. When one is working the other is switched off. This results in crossover distortion at the point when the devices switch.

In Class AB it's like Class B but the "resting" device isn't ever fully switched off. This reduces the crossover distortion.

Is the crossover distortion of class AB amplifiers actually audible?

Charles1dad, Yes it is. You have to keep in mind though that the ear hears many distortions as tonality issues. Also, the presence of distortion tends to obscure detail so in the range where the distortion appears the amp will likely sound harsher and less detailed.

In Class AB it's like Class B but the "resting" device isn't ever fully switched off. This reduces the crossover distortion.

The above statement is incorrect. If the device does not go into cutoff it is by definition a class A device. If it is class AB, the output tube or transistor will stop conducting during part of the waveform.