Here is a description of compresion pedals
http://answers.yahoo.com/question/index?qid=20080621225936AAG52Gz

Also...

I'm disappointed in you all :eek: ... No mention of the root of killer tone. The Chorus pedal. :D :cool: .. That is a must have pedal (IMHO). Probably more so than overdrive and or distortion. Sometimes a lot of chorus, sometimes a little chorus, regardless, I love the shimmer you get in your tone. :)