A ring modulator gets its name from the way the original analog schematic used a ring of diodes to multiply two signals together. Honestly the schematics is more of a square, but who cares when you can trash your sound beyond recognition in both pleasant and horrible ways?
Ring Mod uses either an internal sine/noise generator or a secondary input as the second signal in the modulation, and allows for versatile transformation of the modulating signal:
Ring Mod shines on it's own, but it was primarily created to power up the different "Snapin Hosts" Kilohearts offer. These are bigger effects units that utilize all the available snapins and give you a really fun and creative workflow for combining them in any way you can think of. So have a look at Snap Heap and Multipass right now.
Ring Mod is also available as a Rack Extension for Reason over at the Propellerhead shop.