A song is a metrical composition intended or adapted for singing, especially one in rhymed stanzas; a lyric; a ballad. (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. See part song. (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 "street 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). People sing songs on stage or at a music studio which can go on to the radio or a CD these people are often famous and are very expensive to see live and people go to a live stage which will be on TV.
A song is a piece of music for accompanied or unaccompanied voice or voices or, "the act or art of singing," but the term is generally not used for large vocal forms including opera and oratorio. However, the term is "often found in various figurative and transferred sesnse (e.g. for the lyrical second subject of a sonata...)." The word "song" has the same etymological root as the verb "to sing" and the OED defines the word to mean "that which is sung". Colloquially, song is sometimes used to refer to any musical composition, including those without vocals. In music styles that are predominantly vocal-based, such as popular music, a composition without vocals may be called a song.