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I've marked all the key moments in a class D way of operation. Class D amplifiers are nothing like any other amplifier class. Class A,B,AB are all analog techniques for amplification, with class B having the highest efficiency of up to 75%. Even at 75% the output devices must have quite large heatsinks for everything above 100W. Class D amplifiers have an efficiency of a 100% (theoretically). In reality most class D amps work at around 95% efficiency. They're primarily used when these things are required - mobility (class D implementations tend to be smaller), Efficiency (mostly at high powers to minimize heat losses, or when the amp must work from a battery). On the other hand they are not very Hi-Fi and are very complex if you want them stable and of high quality.
How they work, you ask? Well first of all they're digital, which means that the analog signal must be converted into a digital signal. This is done by sampling the analog signal with sawtooth waves at a very high rate (sometimes in the Mhz range) and turning them to PWM (Pulse Wide Modulation) by using a comparator to track the intersection points of the two signals (sawtooth,analog input). The output result is a square wave which always has the same high frequency of the sawtooth but with a variable duty cycle, the variation being proportional to the input. Once we have the PWM we just need to run it through an LC low pass filter to restore the original signal. As you can see the original signal is quite distorted and very harmonics rich, which can be partially improved, by either using a several order Chebyshev low pass filters, or/and by increasing the sampling frequency.
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