Here is a good article on wikipedia on Sheet Music:
http://en.wikipedia.org/wiki/Sheet_music
An excerpt:
Sheet music is a handwritten or printed form of music notation that uses modern musical symbols; like its analogs — books, pamphlets, etc. — the medium of sheet music typically is paper (or, in earlier times, parchment), although the access to musical notation in recent years includes also presentation on computer screens. Use of the term "sheet" is intended to differentiate music on paper from an audio presentation, as in a sound recording, broadcast or live performance, which may involve video as well. In everyday use, "sheet music" (or simply "music") can refer to the print publication of commercial music in conjunction with the release of a new film, show, record album, or other special or popular event which involves music.
