If you are talking about microphone preamps, they take very small signals (on the order of millivolts) and apply gain to bring them to line level (on the order of a volt or so).
The input impedance is optimzed for microphones (low impedance) but some have instrument inputs that are optimized for guitars and basses (high impedance).
There are a lot more esoteric details that could be covered, but are probably only of interest if you are looking for a certain 'sound'.
