After learning the difference between OD and distortion, I'm curious...

Compression vs. sustain?

They're always grouped together.

I can assume that sustain just lets each note ring longer (not sure exactly HOW, if anyone's up to explaining that one)

But what is compression?
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Compression is basically making the volume of every note played identical. Quiet notes are made louder and louder notes are made quieter so it all comes out equal. It's a unique effect if you have the time to tweak it.
Compression "compresses" the signal. It boosts the volume of a quiet signal and decreases the volume of a loud signal. This has the side-effects of increasing sustain.
Sustain is how long a note rings for before it decays

Compression has allready been explained, you set a certain threshold level and it makes whateve is above the level quiet and whatever is below the level louder so theres less dynamics and notes can ring about a bit more.

The 2 are kind of different terms as sustain happens naturally (eg: "This guitar can sustain for days") where as compression is an effect (though you can describe amps as having a "compressed" sound on the dirty channel or things like that too)