How about an
overly simplistic analogy?
Class A amp is like the 40W light bulb turned on. It is always on, always using 40W, hence hot and inefficient.
Class B is like the 40W light bulb with a manual switch. You can turn it on and off but there is that 'lag' time between on and off.
Class A/B is like a light bulb on a dimmer switch. A little current is always flowing although the light may technically be "off" (dark room with dimmer switch set way down low) which when necessary can go up to its full 40W and down again (quickly move dimmer switch up to full position).
Class AA/B is like the previous description except that its minimum current is usually higher such that the light bulb is always at say a full 5W all the time (the Class A part) but can bump up to 40W and back down again when necessary (the A/B part).
For some definitions of terms,
this is a good site.
For the amp stuff again,
this page is pretty easy.
