What is a Compressor?
A Compressor is a programmable gain(increase in signal strength or intensity) controller designed to decrease the difference in level between quiet and loud signals passing through it. The official definition is a programmable amplifier designed to decrease the difference in level between quiet and loud signals passing through it.
What is a Limiter?
A Limiter is a gain controller whose output goes so high but no further, and above a selectable point any increase in level at its input is not reflected in any increase in level at its output.
How do they Relate to Each Other?
These two functions are related, and can often come together. If dynamic range were a room then a compressor adjusts both the floor and the ceiling, and a limiter adjusts only the ceiling.