Gain - since a compressor can only reduce the volume when the incoming signal is too high, the Gain (output volume) control lets you bring the compressed audio back up to an acceptable level.
Attack time - This is how quickly the volume is reduced once the input exceeds the threshold. If it's too slow, then a short burst of loud music can get through and possibly cause distortion. So when using a compressor as a tool to prevent overload you generally want a very fast attack time
Release time - This determines how quickly the volume comes back up when the input is no longer above the threshold. If it's too fast you'll hear the volume as it goes up and down. That sound is called "pumping" or "breathing." If you don't want to hear the compressor work, set the release time fairly long - one second or more. If you want an "aggressive" sound use a shorter release time. Note that as the release time is made shorter, distortion increases at low frequencies.
Threshold - also called ceiling - This sets the point at which the automatic volume reduction kicks in. Below that volume the compressor does nothing. When the input gets above that level, the compressor reduces the volume automatically to keep the signal from getting much louder.
Compression ratio - This dictates how much the volume is reduced versus how far above the threshold the signal is. A ratio of 1:1 does nothing. 2:1 means if the input rises to 2 dB above the threshold, the compressor will reduce the level by only 1 dB so the output will now be 1 dB louder. 10:1 means the signal must be 10 dB above the threshold for the output to increase by 1 dB. When a compressor is used with a high ratio - say, 5:1 or greater - it is considered a limiter. In fact, the compression ratio is the only distinction between a compressor and a limiter.
