A song may be for a solo singer, a lead singer supported by background singers, a duet, trio, or larger ensemble involving more voices singing in harmony, although the term is generally not used for large classical music vocal forms including opera and oratorio, which use terms such as aria and recitative instead. Songs with more than one voice to a part singing in polyphony or harmony are considered choral works. Songs can be broadly divided into many different forms, depending on the criteria used.
Songs may be written for one or more singers to sing without instrumental accompaniment or they may be written for performance with instrumental accompaniment. The accompaniment used for a song depends on the genre of music and, in classical styles, the instructions of the composer as set out in the musical score. Songs may be accompanied by a single accompanist playing piano or guitar, by a small ensemble .