Turntable/Record Player cartridges output signals at a much lower levels than CD Players etc. Many Amplifiers have extra Amplification Built-in to compensate for this low input signal (Phono Input) . Many Modern Amplifiers and Digital Surround Receivers don't have ‘Phono' Inputs so an Offboard Phono Preamplifier is required (see Line Level).
A special input found on most amplifiers that can receive signals from a record player. The signal frequency of a record player is lower than that of other sources (CD, Tuner, etc.) and therefore requires this special input. Amplifiers without a Phono Stage built-in need an external phono stage to convert the record player output into a signal that the amplifier can handle.
Cartridges output signals at much lower levels than CD players and tape decks. Many amps have the extra amplification built-in, but increasing numbers don't, and require an add-on phono amplifier.