A song is a musical composition. Songs contain vocal parts that are performed with the human voice and generally feature words (lyrics), commonly accompanied by other musical instruments (exceptions would be a cappella and scat songs). The words of songs are typically of a poetic, rhyming nature, although they may be religious verses or free prose.
Songs are typically for a solo singer, though there may also be a duet, trio, or more voices (works with more than one voice to a part, however, are considered choral). Songs can be broadly divided into many different forms, depending on the criteria used. One division is between "art songs", "popular music songs", and "folk songs". Other common methods of classification are by purpose (sacred vs secular), by style (dance, ballad, Lieder, etc), or by time of origin (Renaissance, Contemporary, etc).
Colloquially, song is sometimes used as slang to refer to any music composition, even those without vocals. In European classical music, jazz, brass band, popular music, and many other musical styles however, this usage is considered incorrect. "Song" should only be used to describe a composition for the human vocals. In music styles that are predominantly vocal-based, a composition without vocals is often called an instrumental. A musical piece that may be either with or without vocals can be called a melody, a tune, or a composition.
