Class A/B/C/D amplifier

In spite of modern digital technology, transistors as well as tubes are still much in the same way. Certain characters may accomodate for different purposes, by using different amplifier classes.

Class A

The amplifier conducts 100% of the signal period. The effiency is low, not more than 25% at full load, and lower with lower output. There is no crossover distortion. Class A can be used in small signal circuits where power dissipation is negligible, yet to obtain optimal linearity.
Examples of use are high end headphone amplifiers and audio power amplifiers.

Class B

Is conducting 50% of the time and uses two complementary active elements that conduct one at a time (push-pull). Class B amplifiers are subject to crossover distortion if there is any mismatch, hence a notch distortion might occur unless the bias for each of the element is shifted which would cause an overlap as in class AB.
Today nearly all stereo amplifiers use class B yet it is difficult, if not impossible, to measure any switching distortion.

Class AB

Class AB is a combination of A and B. It was created to leave one element conducting while the other was conducting. This may create an overlap between the two signals. Hence, the signal would become larger in the overlap. The efficiency is higher than of class A and lower than of class B.

Class C

Is conducting less than 50% of the signal period. A tuned filter after can filter out the distortion. It is mostly used in RF applications where a linear amplification is not neccessary such as in FM transmitters. However the voltage can be AM-modulated, hence the RF output too, with fairly low distortion.

Class D

Is a switching amplifier. Although the circuitry performs on-off switching, or PWM (pulse width modulation), the "D" does not stand for digital.
The principle can be described with a pulse width, being "on" 25% of a certain period length, the output becomes + or -0.5 of a full signal. If the pulse width is 50%, no output will occur. The switching speed has to be at least twice the highest signal frequency but most designs utilize a much higher sampling rate. The effiency can easily be 90% and since it will not develop as much heat as other amplifiers, smaller heatsinks can be used.

Links:

Amplifier classes - simplified overview
Amplifier classes - Duncan's amp pages