compression on guitars etc.

[michaelSHK]

I think an important thing to make clear is that a compressor doesn't "turn down" the volume. It reduces the dynamic range of a signal. The more compression, the less difference there will be between the loudest and quietest part of the signal.

Google ADSR, and read a little bit about envelope. A compressor gives you some amount of control over a sound's envelope, or, a waveform's changes in amplitude over time.