Audio Dynamics 101: Compressors, Limiters, Expanders, and Gates:

In this article, we’ll demystify the compressor and other dynamics processors. We’ll cover the four main types of plug-ins used to control dynamics: limiters, compressors, expanders, and gates. We’ll discuss the mathematical processes behind these tools, how they affect the sound, and the best scenarios in which to use them.

Expanders are my least understood tool. Gates are kind of troublesome as well. This is a good description of what they do.