ive heard the term, but i never understood what they mean, and what the differences really are. i know its a noob-ish question but i would really like to know.

on an unrelated topic, whats the difference between the little big muff and the big muff other than size... is the tone the same?

thanks :peace
A power amp supplies the power to drive your speakers, monitors, etc. A preamp takes in your original signal, and prepares it for the power amp. THe preamp contains the EQ, tone controls, etc.
The preamp is the first bit of the amp. It amplifies the signal just a little bit so's the power amp can bring it up enough to power the speakers. In a tube amp, you'd probably have 12AX7 tubes in this part, as well as other 12A_7 tubes to drive reverb or tremolo if you've got it. The EQ and gain controls all lie within here.

The power amp is the second part of the amp. It brings the very weak signal from the preamp up to speaker-driving level, often using EL84s, EL34s, 6L6s, 6550s, 5881s, 6V6s, KT66s, KT77s, etc.

You can also buy each part separately for use in a rack system, if that's how you roll.