A song is poetry set to music. It can be as simple as a happy birthday tune, or as complex as an art song with an accompanying musical accompaniment. Usually, a song is for a single singer, although it can be for multiple voices, as in a choir or an opera.
It is sometimes difficult to define a song, but there are some criteria that most songs must satisfy:
Verses tell the story of the song and should support the chorus by moving forward. The chorus is the big payoff and climax of the song, so the verses and pre-chorus should build up to it, either musically or lyrically. The chorus can also contain a hook, which is the catchiest melody in the song.
A bridge is typically one of the first things in the song to change the pace or emotion, or both, and it serves as a transition between the choruses. It is usually a bit shorter than the verses and the pre-chorus, and may repeat the same melody or lyrics.
Scholars have studied songs in many cultures, and they find that acoustic features of music predict the primary context of its performance (see the Perspective by Mehr and Popescu). They also report that the melodies and rhythms of most songs are regular, and that elements of these patterns are found worldwide, suggesting that some kind of universality may exist. Nonetheless, there is considerable within-society variation in music, as well as between societies, reflecting the different ways that people perceive and enjoy music.