A song is a musical composition that contains vocal parts ("lyrics") that are performed ("sung"), commonly accompanied by musical instruments (exceptions would be a cappella songs). The lyrics 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 they may also be in the form of a duet, trio, or composition involving 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", "pop songs", and "folk songs". Other common methods of classification are by purpose (sacred vs secular), by style (dance, ballad, Lied, etc), or by time of origin (Renaissance, Contemporary, etc).

Colloquially, song is sometimes used to refer to any musical composition, including those without vocals, however this usage is, by definition, incorrect. In music styles that are predominantly vocal-based, such as popular music, a composition without vocals may be called a song but is often called an instrumental.

Art songs

Art Song are songs created for performance in their own right, usually with piano accompaniment, although they can also have other types of accompaniment such as an orchestra or string quartet, and are always notated. Generally they have an identified author(s) and composer and require voice training for acceptable performances. German-speaking communities use the term art song ("Kunstlied") to distinguish "serious" compositions from folk song ("Volkslied"). The lyrics are often written by a poet or lyricist and the music separately by a composer. Art songs may be more formally complicated than popular or folk songs, though many early Lieder by the likes of Franz Schubert are in simple strophic form. They are often important to national identity.

Art songs feature in many European cultures, including but not limited to: Russian (romansy), German (Lieder), Italian (canzoni), French (mélodies), Scandinavian (sånger), Spanish (canciones). There are also highly regarded British and American art songs in the English language. Cultures outside of Europe that have a classical music tradition, such as India, may or may not feature art songs.

The accompaniment of European art songs is considered as an important part of the composition. The art song of the period in which they originally flowered is often a duet in which the vocalist and accompanist share in interpretive importance. The pieces were most often written to be performed in a home or salon setting, although today the works enjoy popularity as concert pieces. The emergence of poetry during this era was much of what inspired the creation of these pieces by Brahms, Schumann, Schubert and other composers. These composers set poems in their native language. Many works were inspired by Johann Wolfgang von Goethe and Heinrich Heine. Another method would be to write new music for each stanza to create a unique form; this was through-composed form known in German as durchkomponiert. A combination of both of these techniques in a single setting was called a modified strophic form. Often romantic art songs sharing similar elements were grouped as a song cycle. (Kamien, 217–18)