what is a preamp and a power amp?
How and why would u use one?
how do u set it up?
All guitar amps are made up of a pre-amp and a power-amp. The pre-amp is where the guitar signal is processed and any gain/EQ is added. The power amp is where the signal is actually amplified up to the level that it comes out of the speakers.
You can buy pre-amps and power-amps seperately (normally set-up in a rack), allowing you to mix and match as you see fit. However this is a lot less common, and generally best left to professionals and experienced guitarists that know exactly what they want.