A pre amp is a low level amplifier meant to be the first stage in amplification. It generally brings to signal up to “line” or “aux” level. It is not intended to drive speakers. Pre amps are often used to connect to a mixer, the mixer in turn feeding a power amp.

A power amp is generally meant to accept the line or aux levels in and amplify them to drive speakers.

Some amplifiers are both in one box. You can connect your instrument to the input and speakers to the output.