Cassette (bicycle part)

From Freepedia

On a bicycle, the cassette is the set of rear sprockets that attaches to a freehub on the rear wheel. It is a later development of a cogset. Cassettes are distinguished from freewheels in that a cassette typically has a series of straight splines that form the mechanical connection between the gears and the hub. The entire cassette is retained on the hub by means of a screwdown locking ring.