limiting and gating

Limiting is primarily used when you want to turn something up but the peaks are clipping (going into "the red" and distorting). You can use it anywhere that you have that problem. Compression is similar to limiting but affects the whole waveform more gradually.

Gating at its most basic is used to cut out "noise between the notes"-- e.g. guitar hum. It can also be used to cut short the sustain of notes to make them snappier (e.g. kick or snare) or to reduce the level of "room sound." Sometimes a little gating get help a slightly "sloppy" sounding track if there are lots of breath noises or squeaks and creaks between the notes. A little goes a long way, and very often some gentle expansion works just as well as hard gating without getting the "chopped off" sound of a hard noise gate.

Cheers.