There are a number of variations to the compression scheme.

A limiter is a compressor with a fast attack, a high compression ratio and are used with a high threshold (ie only the loudest peaks are affected.

Side band compression is used to control the loudness of one signal by another. A typical applications is the use in voice over. The loudness of the voice signal when present, is used to decrease the loudness of background music to ensure that the music doesn't swamp the voice.

A De-esser is used to control the sibilance of a voice. Works by only feeding the control the the sibilant parts of the sound.

A Noise Gate works like a compressor in reverse, making the quietest part of the signal quieter still. This assumes that the quietest parts of the signal are purely noise.

Try the demo program compressor .