Most home theater components are simple and named after their functions, here are the more complicated devices:
Receiver - receives a digital or analogue sound signal from one or more audio sources (such as TVs, game consoles, computerss, media players, etc) and processes the sources into a high voltage speaker signal. Receivers these days typically support 8 or more speakers (eg; 7.1 - 7 speakers and a subwoofer)
Switch - allows you to plug multiple video sources and a video output into it and control which source you wish to view, usually with a switch, dial, or multiple buttons.
HTPC - Home Theater PC, a computer built for the purpose of playing a library of movies, games, music and more from a built in or external hard drive. Modern HTPCs also include BluRay drives to eliminate the need of a dedicated BluRay or DVD player.
More information can be found here:
http://en.wikipedia.org/wiki/Home_cinema