Cassettes are a cluster of sprockets in a range of sizes offering a multitude of gear ratios. A cassette slots onto the freehub, which has the ratchet built in and is then secured with a lockring.  With cassettes you must use the appropriate spacing for your gear levers and rear mech.

Cassettes come in many ratios,
 Close ratio is when the number of teeth increase in small increments of 1 or 2 at a time,
 Wide ratio is when the teeth increase in larger increments of 3 or 4 at a time.

Cassettes

Road Cassettes MTB Cassettes