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Phono (turntable) input
When a turntable plays a vinyl record, it produces an audio signal that's considerably different from the standard line-level signal output by CD players, tape decks, and other components. Not only is the phono signal much lower in voltage, but the process of vinyl mastering involves applying a special equalization curve that cuts low frequencies and boosts highs. (This prevents the "overcutting" of grooves, and minimizes noise.)

Many receivers feature a specialized phono input intended for turntables only. The signal coming from this input is sent to a dedicated preamp circuit which applies inverse equalization and boosts the signal up to the standard line level — resulting in proper sound from your turntable. (Keep in mind that feeding an ordinary line-level signal into your receiver's phono input will probably result in horrific distortion, and could actually damage your amplifier and speakers!)

Receivers with a phono input also feature a ground screw for connecting your turntable's ground wire. (Neglecting this connection can result in an audible hum from your system.) If your receiver or preamplifier doesn't have a specialized phono input, you can use an in-line phono preamp, which allows you to connect your turntable any regular line-level input.