I couldn't find a good link, but can explain the basics to get you started:
Most amps are class A/B. Class A means that there is always current in the output stage going to the speakers, so long as the amp is on (even if no signal from the input stage). At higher levels, additional current needs to be turned on (Class B). However, it is this switching on and off that is inefficient. An amplifier functions best when it is either 100% on or 100% off. Anything in between is usually wasting a lot of energy as heat.
All on or all off ... 1 or 0 ... sounds like digital! Indeed, a digital amp works by controlling the switching between the current stages. In the same way that if you sample at a high enough frequency, it sounds continuous, if you switch the higher current on and off fast enough, it sounds continuous. My amp, which is a digital switching amp (PS Audio HCA-2), switches at 500 kHz - way fast enough!
The input and output stages are still the same as many other amps, but the current control is what changes (that's the digital part - the on/off current switch).
HTH. If you want to listen to one, let me know