Poetry Guide: Poem and Song
Where verse is set to music, the distinction between poem and song may become artificial — to the point of being untenable. This is perhaps recognised in the way popular songs have lyrics :. The verse, however, may precede in time the tune (in the way that "Rule Britannia" was set to music, and "And did those feet in ancient time" has become the hymn "Jerusalem"); the tune may be lost over time but the words survive; a number of alternate tunes may fit (this is particularly common with hymns and ballads).
Possible classifications proliferate (under anthem, ballad, blues, carol, folk song, hymn, libretto, lied, lullaby, march, praise song, round, spiritual). Nursery rhymes may be songs, or doggerel : the term doesn't imply a distinction. The ghazal is a sung form that is considered primarily poetic. See rapping, roots of hip hop music also, on the boundaries: verse+music against verse against verse set to music.