Unlike line level audio sources you connect to a stereo (DVD/CD players, tape decks, TV audio, minidisc, etc.), the output from a magnetic cartridge installed in a good quality turntable is MUCH lower, and requires an additional stage of amplification to bring it up to the same volume as the other sources you listen to thru your stereo.
This additional amp stage, the phono preamp, is built-in to most older receivers and amps, allowing direct connection of a turntable. However, newer stereo equipment (including virtually all mini-systems and home theatre units, as well as many stereo receivers and amps), have NO phono input (this because records and turntables are supposedly obsolete in today's world dominated by CDs and DVDs).
In order to utilise the inputs such units DO have (Aux, Tape, Line, Video, CD, etc.) to connect a turntable, you need to first pass the signal thru an external phono preamp to bump the level. The same level increase is needed if you're connecting a turntable to a computer sound card's line input so you can make CD-Rs from LPs; again, the external phono preamp provides it.
Because of limitations in the LP recording process, an equalization curve must be applied to the music or other sonic content prior to it being cut onto vinyl, so as to reduce backround noise and sibilance. Reversing this equalization effect (the RIAA curve) and restoring the music's original frequency response curve during playback is an important part of the phono preamp's job and differentiates it from other preamps used for microphones and musical instruments, which usually provide gain but no other modification of the original sound quality. Proper RIAA re-equalization during playback is a must in faithfully producing the original musical content without coloration or distortion.