This refers to a Amplifier, that uses a NPN transistor and a PNP transistor. The transistors are matched so they roughly have the same gain, speed and current specs. The collector of the NPN is attached to V+ and the collector of the PNP is attached to ground. The emitters of the two transistors are commoned and go to the positive side of a capacitor. The two bases of the transistors are also connected together thru usually with two diodes in between to match the .7 voltage drop from base to emitter. When the signal is positive the NPN transistor comes on and pulls the capacitor up to closer to V+, and when the signal goes low the NPN closes of and the PNP comes on and drains the voltage to ground from the capacitor.
Example: Class B Push Pull Amp:
