Hi, good info on compressors at
www.dbxpro.com/white.htm but here's a quick overview.
It is useful to think of a compressor as a sort of automatic volume control. When the signal being compressed exceeds a target level (the threshold), the volume is reduced by some proportion (the ratio). There are other controls which determine how quickly the reduction occurs (Attack) and how quickly the original volume is restored after the level drops below the threshold (Release).
Many/most compressors have another control to raise the average output level (Makeup Gain).
The most common application for compressors is to keep transient peaks (Volume spikes) from causing distortion due to clipping or digital overs.
They are also used to raise the apparent loudness of a track by reducing the difference between the loudest and quietest parts (the dynamic range).
Effects processors are used to add things like reverb/delay, etc. to a track.
In general there are three types of signal processing - dynamic processing (compressors/limiters/noise gates/expanders), Effects processing (reverb, echo, flange, etc), and EQ.
[This message has been edited by Larry Sheehan (edited 11-19-1999).]