Silly question...is there a Class C, and if not, why did 'they' skip a letter?
The various amplifier classes just represent different ways of building amp circuits, and are more-or-less consecutive ... ie Class A is the simplest and was used at the turn of the 20th Century.
This is a simple explanation, but if that's all you want it's enough to get the concept.
Broadly speaking if you go down the list from A to B to C to D, the major refinement is all about better efficiency. With A, you can use one device (transistor, tube) to do all the work. It's "on" to some extent all the time.
With Class B, you use two devices and one works on the positive side of the signal, and hands off to the other which works on the negative side. When one device is working, the other rests. Class C is similar to Class B, but the two devices do not conduct during the entire 1/2 wave; both are "off" during a time when in Class B one would still be doing work. Again, the less time the device is doing work, the greater the efficiency. But with Class C, there is a gap where neither device is doing any work ... you can't really get a true sine wave out of a Class C device, so it's unsuitable for audio.
Class B is also somewhat unsuitable for audio, but by giving up a bit of efficiency and operating in Class A for small signal levels, it's inherent problems can be addressed. So, for audio, it's usually what is called Class AB; it still operates in Class B for most of the time and still gains some efficiency that way.
Class C is used in transmitting devices where the need for very high power means the efficiency is prized. Because RF amplifiers use fairly complex filtering anyway as they must work at a specific frequency, the filters can mitigate some of the distortion inherent in the design. There is also Classes E and F. Any other "classes" you might hear about are probably invented by the marketing department; the engineering department would probably put them in A, AB, or D.
Although I said earlier in my post that Class C is not suitable for audio, there is such a thing as a Class C audio amplifier in consumer products. Again, it's a situation where power and efficiency are the priority over sound quality. You have heard one of these devices, so you will quickly get a handle on it's characteristic sound.
Typically outputting 5~25 watts under battery power, Class C is commonly used in powered or electronic megaphones. I suspect, but don't know for sure, that some Public Address or Stadium sound systems use class C (or perhaps straight Class B) but I don't know of any examples and can't say for sure either way.
