Class D is basically a switching amp as opposed to a liner amp. The mains is smoothed and then switched by a digital PWM (Pulse Width Modulated) signal representing the incoming audio signal to switch the output MOSFETS. The switched signal then passes through a low-pass filter to remove the switching component and restore the original, but amplified audio signal
Class D has become very popular in large PA amplifiers as it allows for high power in a small chassis as there are no power transformers and the MOSFETS are either fully on or fully off at any point in time so generate little heat and are thus way more efficient than conventional amplifiers.
Crown amplifiers (Alan's Favorite Amps... ?) has a derivative of Class D called Class I. Without getting into the technicalities, Class I offers even more power and efficiency than Class D.
How about 4500w/channel in an amplifier that only weighs about 12kg and draws only 8 amps max.