what exactly is a ring modulator?

also is that the effect heard in the intro on David Bowie's "Kooks" from the Hunky Dory album? or what is that?
Its apparently what Iommi used in the solo of paranoid to give it that edge. But arent many companies who make em. Last 1 I saw was 250 bucks. Theres one in my MFX pedal but doesnt have any usable settings. The moog one has a bunch of knobs so would take a little bit of time to figure out.