5.1 refers to the total number of speakers.
two in front on the sides.
one center
two behind you on the sides
.1 refers usually to the subwoofer.
as mentioned your reciever\amplifier is a hub. Sound from the television, or cd\mp3 player, or DVD player that you want amplified loud out of the speakers goes through the reciver\amplifier using an assortment of overpriced cables.
as "components" (things that you plug into the receiver) have become more complex and computer~ish the receivers have become smarter themselves. In my experience, if you are restrained by prices, you can afford to be cheap on one of the two. Either cheap amp. Or you can cheap on speakers. One good quality of those two will USUALLY and sufficiently pick up the slack for the other.