A home theater is pretty much just what it says, a theater in your home.
There are really five main components associated with a home theater:
A center speaker sits above or below the viewing area. It is usually positioned in the center (of course) and is aimed directly at the audience. The main job of the center speaker, contrary to many people's beliefs, is to carry all on screen sounds, this includes explosions, all dialog, shooting, booms bangs, and any thing else that passes by. Fifty two percent of your total home theater experience is from the center speaker, undoubtedly the most important speaker in the HT.
The second most important speakers in you theater are your left and right speakers. These speakers carry the weight of about 32% of your total experience. The left and right speakers should be placed next to your viewing area. If using a t.v. take the measurement of it horizontally. Then multiply that measurement by four. This should give you the size of your soundstage, from your right speaker to your left.
Ah, the surround speakers, I must say that they are my overall favorite. Probably the leaste important speakers of the group the surround speakers transmit time delayed data from the audio sorce. They create a three deminsional enviroment instead of just the old two deminsional. They, to me, really add the effect of being there. These speakers should be placed two both sides of the listening area. About seven feet from the listener on both sides and even with hearing level. For the most part the surround speakers can be small for they are not needed for a large ammount of bass. At least the weren't. Now there is Dolby Digital, DTS, and SDDS that requieres full frequency from all speakers, excluding sub. But no matter what decoding system you are using the surround speakers are awesome!
The subwoofer is a separate speaker designed to handle the lowest bass frequencies, the powered subwoofer does more than just reproduce those very low noices. It also improves the sound of midrange and upper bass frequencies by freeing the main stereo speakers and amplifier from the stresses of reproducing powerful bass.