Originally posted by Christoph

Everything . . . what it is, what it does, how it works, settings to use, etc.

what it does => in most cases it attenuates all levels that go above a set threshold, by a pre-(user)determined ratio. example: you have a threshold of 10dB, and a ratio of 2:1. Now, if you have a input level of 20dB, thus 10dB over the threshold, the compressor attenuates that level by 2:1, thus bringing it down to 10dB.
In short, it decreases the dynamic range of the original input signal.

how it works => I have no clue, probably with lots of resistors, capacitors and voltage controlled amplifiers. This part is too techie for me.

settings to use: Can't help you on that one either, I never pay attention to my settings, whatever sounds good goes.