what does this pedal exactly do,technically? I was looking through the boss site and I was listening to the sound clips, it kind of sounds like a chorus pedal to me. I've heard John Frusciante loves using these and i was just curious.
it makes the louder part of your playing softer and the softer parts of you playing louder.
Gibson Les Paul Studio & mexican strat.
Crybaby wah (crappy)
pod xt live
Marshall valvestate combo
John Frusciante does not use a compressor pedal, but has alot of compression in his sound. This comes from on the board (ala BSSM) and live comes from natural compression from his tube amps and strat combo at loud volumes. A best example of this sound would be Can't Stop's main riff, or Give it Away.

Another thing it does is even out chords and make Jimi Hendrix-like playing alot smoother. It makes all the notes in the chord the same volume so it sounds alot better, and the lows don't over power the highs or vice versa. Plus, a chord in totallity will be the same volume as a single note, so if you're doing some Hendrix playing, blending between chords and notes, it sounds alot better.

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A compression pedal makes all of your picking sound at the same frequency. When you use a compressor, theres no dynamics in your sound. Soft picking and hard picking are compressed to sound like the same picking strength.
if you imagine sound waves, with the higher volumes going up further and the lower volumes going not as high up, what the compressor pedal does is to push all these waves together, creating more of a flat line (the amount of flattening is usually variable). this can make your playing sound smoother and less eratic.
I think it'll also smooth out the signal if you are having problems with the tone after the signal goes through a million pedals.