A cluster of sprockets, commonly 5 to 11 of them, attached to a freehub on the rear wheel.
Multi-geared bicycles often use a 'cassette' of sprockets which slide onto a splined freehub body. This system has largely replaced the screw-on cogset used on older models. There are usually between 5 and 11 sprockets in a cassette, and ratios vary widely based on intended application.
Cassettes are referred to as a Block by the English, more so amongst the older generations.