The Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. Although designed originally for dictation, improvements in fidelity led the Compact Cassette to supplant the Stereo 8-track cartridge and reel-to-reel tape recording in most non-professional applications.[1] Its uses ranged from portable audio to home recording to data storage for early microcomputers. Between the early 1970s and the late 1990s, the cassette was one of the two most common formats for prerecorded music, first alongside the LP record and later the Compact Disc.[2]

Compact Cassettes consist of two miniature spools, between which a magnetically coated plastic tape is passed and wound. These spools and their attendant parts are held inside a protective plastic shell. Two stereo pairs of tracks (four total) or two monaural analog audio tracks are available on the tape; one stereo pair or one monophonic track is played or recorded when the tape is moving in one direction and the second pair when moving in the other direction. This reversal is achieved either by manually flipping the cassette or by having the machine itself change the direction of tape movement (“auto-reverse”).[3]

more at http://en.wikipedia.org/wiki/Audio_cassette_tape