overdrive is what happens when you try to get too much power out of a tube amp, the tubes 'distort' which was known as overdrive. distortion, or fuzz, is actually an effect in which i think you maximize the high and low peaks of the sound wave giving either a triangle or square wave instead of a smooth wave creating the fuzzy sound we know as distortion.. there are overdrive pedals, but i believe they just try to emulate the overdriven tube amp sound..

i'm not an expert so i might be wrong on some of this, but i think i got most of it right

jason