Expanders are dynamic processors that increase the dynamic range of a signal. They make loud sounds louder and soft sounds softer. They're like an amplifier with a variable gain control and the gain is controlled by the level of the input signal. Expanders are used by musicians to produce more extremes in recordings that have a limited dynamic range. This gives the music more intense dynamics.

Noise gates are typically used to eliminate unwanted noise and hiss which would otherwise be heard when an instrument is not being played. The noise gate's threshold needs to be at the precise point so unwanted noise falls below it and the instrument's complete notes can be heard.

Expanders and noise gates should be near the end of the effects chain, so noise created by other effects can be "gated" when you're not playing. I place my delay and reverb effects after the noise gate so their sounds trail off naturally.