Full Bridge Class D Amplifier. A triangular wave is compared against the input Signal. The result is a PWM Signal and thereby digitized. The triangular wave usually has a high frequency (e.g. 250kHz). As a result the output Transistors just need to change between "on" and "off" minimizing losses. Due to the high frequency of the PWM signal it's possible to remove the high frequency components with a low-pass(in the simplest case the Speaker itself) and retrieve the amplified input Signal.
The Diodes placed in parallel to the MOSFETs are used to reduce the switch-off time.
The configuration shown is a Full Bridge to increase Power output at a given operating Voltage. The principle is comparable to the H-Bridge used in DC-Motor Circuits
