Ring modulators

can anyone explain ring modulators to me? i hear them all the time in tool and incubus songs, and i love the sound of the DOD gonkulator modulator, but i dont understand what the pedal is really doing, just that it screws up the note. i specifically want to know about gonkulators because im thinking of buying one.

here you go:

http://www.harmony-central.com/Effects/Articles/Ring_Modulation/

wow... that explains a lot. thx. now if only i could find one of those gonkulators for sale...