A song is a musical composition performed by the human voice. The voice often carries the melody (a series of distinct and fixed pitches) using patterns of sound and silence. Songs have a structure, such as the common ABA form, and are usually made of sections that are repeated or performed with variation later. A song without instruments is said to be a cappella.

Written words created specifically for music, or for which music is specifically created, are called lyrics. If a pre-existing poem is set to composed music in the classical tradition, it is called an art song. Songs that are sung on repeated pitches without distinct contours and patterns that rise and fall are called chants. Songs composed in a simple style that are learned informally by ear are often referred to as folk songs. Songs composed for the mass market, designed to be sung by professional singers who sell their recordings or live shows, are called popular songs. These songs, which have broad appeal, are often composed by professional songwriters, composers, and lyricists; art songs are composed by trained classical composers for concert or recital performances. Songs are performed in studios and an audio recording is made, or they are performed live for audience. (In some cases a song may be performed live and simultaneously recorded.) Songs may also appear in theatre (e.g., opera), films and TV shows.

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. A song can be sung without accompaniment by instrumentalists (a cappella) or accompanied by instruments. In popular music, a singer may perform with an acoustic guitarist, pianist, organist, accordionist, or a backing band. In jazz, a singer may perform with a single pianist, a small combo (such as a trio or quartet), or with a big band. A Classical singer may perform with a single pianist, a small ensemble, or an orchestra. In jazz and blues, singers often learn songs by ear and they may improvise some melody lines. In Classical music, melodies are written by composers in sheet music format, so singers learn to read music.

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 and types, depending on the criteria used. Through semantic widening, a broader sense of the word "song" may refer to instrumentals, such as the 19th century Songs Without Words pieces for solo piano.