In reply to:

One thing I have never understood is the difference between Receivers and Amplifiers


Again, there's some leeway in nonclamenture here.

Most simply:
A receiver is what you're used to seeing, it has a radio tuner and switches audio sources at line level, amplifies them and sends them out to the speakers.

An A/V receiver is the same, but it also switches video sources.

An amp simply amplifies the signal, no source switching.

Bren R.