Josh, the preamp, in whatever configuration, sends a signal to a power amp(s). the power amp does just that, amplifies the signal that it is given, into 'X' amount of output to the speakers.
The preamp does all the switching, and in some cases, the decoding of signals(a processor), although these lines are somewhat blurry at the moment with various inputs and so on.
A Power amp has low level inputs, and produces high power outputs at the speaker terminals.
Sysyems like this are termed as 'seperates', as compare to Integrated Amplifiers, which combine the two componants.
It does get even more complicated when you bring in processors i.e. pre/pro. I would suggest starting with the glossary and going from there. You should read about integrated amplifiers as well. All the best.