A receiver consists of 3 basic components:
1. A preampflier. It accepts the line level inputs, and controls the volume, and switching between the various inputs, such as DVD, cable box, cd player, etc. In the case of a home theater preamp, also known as a preamp/processor, or prepro for short, it also decodes the dolby digital, DTS, etc surround modes.
2/ An ampflier, it takes the line level input from the preamp and amplifies it to a level that the speakers can use. it typically has no volume controls, usually only an on/off switch.
3. A tuner, to receive AM/FM radio signals.
As you have seen, you can get these components separately from a receiver, which allows you to mix and match the best features with the proper power for your application. The individual components also have the advantage of separate power supplies, instead of a single power supply providing power for all the components, as in the case of a receiver.