it cancels out hums and feedback when you play at high volumes or record and stuff like that. some amps these days have them built it. i cant recommend a good one, but Rocktron makes a decent one, so ive heard.
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My gear:
Ibanez Artist
ESP ltd Viper 200
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A noise GATE attempts to shuts out ALL your sound when you aren't playing.

A noise supressor attempts to filter out noise while leaving your playing intact.

I;m saying attempts because I have not yet found a gate that doesn't affect your tone, and the only spressor I've found that doesn't affect your tone is an ISP Decimator.
WTLTL 2011