Ever hear someone talk about the cassette on their bike? Or have someone suggest your cassette might need replacing? It's not a tape for playing music, nor related to a videocassette, like most people know the word. The cassette is part of your drivetrain, which is your pedals, crankarms, chain and sprockets. It is the stack of gears on the rear wheel of a bike that a person can shift to make the riding easier on hills or to go faster. The chain moves from one gear to another on your cassette by use of the rear derailleur.