- Preamp: The 'brains' of a system. It switches among various inputs (CD, DVD, Tape, etc). It takes low level ('line level') signals and amplifies them a tiny bit and provides a volume control. Nowadays preamps also include processing for various digital formats (Dolby Digital, DTS, etc) and are generally known as 'pre/pros' (for preamp/processor).
- Amp: amplifies line levels signals and sends them to your speakers.
- Integrated Amp: Preamp+Amp in one box.
- Receiver: Preamp/Processor/Amp+Tuner(radio) all in one box.
For most systems a Receiver is all you need. If you have a large room or want to listen at extremely high levels, a separate amp may be necessary. You can either buy a separate pre/pro and amp or buy a receiver that has pre-outs and connect an amp to the receiver pre-outs.