Also known as a push-pull amplifier, the most basic design consists of two matched transistors in series, an NPN tied to the upper rail, and a PNP tied to the lower rail, with the junction balanced at about ground. The same input signal controls the gates of both transistors. When the signal goes high, the NPN transistor activates, and more power is fed into the load. When the signal goes low, the PNP transistor starts to sink current from the load.

Unlike the inefficient Class A amplifier, the quiescent class B amplifier draws no power. However, there is some distortion introduced, due to the fact that the transistors both need about .7 Volts of offset before they activate. This creates a deadband effect, where small signals do not cause any output at all. The solution to this is to create a bias voltage for both transistors, ususally using some diodes that can offset by the same .7 volts as the transistors. This is sometimes called a Class AB amplifier, but not by me.

Class B amplifiers are usually what are found in your stereo.