Perhaps a dumb question...

To reproduce the sound on an LP you must use either a built-in or stand alone phono preamp. Basically a phono pre-amp does two things. One, it converts the information embedded in the groove according to something called the RIAA (Recording Industry Association of America) curve. The curve is used during the cutting of LP's to compress the signal at its frequency extremes. Two, it amplifies the very low level signal produced by the phono cartridge. A cartridge's stylus riding in the groove generates a very faint signal. If you put your ear close to a record while it's playing you can hear it.

Back when turntables were the only game in town all stereo preamps, integrated amps and receivers had built-in phono pre-amps. When the popularity and availability of LP records waned manufacturers dropped the phono sections from their products. So nowadays systems with turntables use stand alone phono pre-amps.

I have had phono preamp for years but couldn't explain its function. The above explanations are very helpful, thanks audiogoners. There is no dumb question.

But if my receiver or amp has an input that says "Phono" as well as a knob that says "Phono output" or something like that, is it safe to say that it has a built in phono stage?

You better check your owners manual before you assume there is a phono stage built into your receiver. But anyway you'll know for sure when you plug it in. With no preamp you'll get crap out...

We cannot assume that your receiver has a built in phono stage. Some amps just label their inputs for convenience. As Sugarbrie said, the best thing to do is to check the owner's manual. If you can't find the owner's manual, just plug your turntable into the input labeled phono. If nice music comes out, then there is a phono stage. If funny sounding (too low in volume or too tinny with little bass) music comes out, then there is no phono stage. Can you tell us which receiver you are using? Maybe someone has a similar one and can tell you whether it has a phono stage.