Well there is the pre-amp that metalnick spoke of, and he is right about that. And then there is another sort of pre-amp that is external to your amplifier. I have one here - its an ancient "Linear Power Booster" that was made by the ElectroHarmonix company some time back in the seventies. It looks like your typical stomp box - in fact if you have seen any of the new ElectroHarmix re-release stomp boxex in the music stores, they look pretty much the same as the old ones. The linear Power Booster just takes whatever signal that is fed in to it and boosts its volume up. The amount depends on how far you turn the boost knob. What is it used for? Well, heres a couple of uses.
A) You have an electric guitar with humbuckers that dont have very high output. IE they dont SCREAM like you would like them to. You run it through the Linear power booster and it give those pickups more "volts" which will saturate the front end (ie pre amp) of your amplifier, which in turn will give you more distortion.
B) You are trying to mic up an acoustic instrument, but you microphone isnt sending enough volume to the mixing board. Same thing - You place the Linear Power Booster between the mic and the mixing board and it will boost the mics signal level so that you have more headroom for mixing in that accoustic instrument.
The Linear Power Booster does not add any distortion on its own though - it just boosts the levels.
Click here to see a picture of this booster - looks just like mine.
Rj