This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
A receiver is a tuner, power amplifier, and preamp combined. A
common receiver has inputs for a turntable, a CD player, a tape
deck, and perhaps one or two other sources. It probably also
has selector switch(s), tone controls, and a volume control. A
receiver may have outputs for two speakers, or for more. Some
receivers do | not have phono preamps, a trend that may become
more common as vinyl loses popularity. Many receivers contain
surround sound processors.