Ballad

"A narrative, rhythmic saga of a past affair, sometimes romantic and inevitably catastrophic, which is impersonally related, usually with foreshortened lines and simple repeating rhymes, and often with a refrain." Packard, The Art of Poetry Writing 93

Ballads are most often folk poetry in a musical format, passed along orally from generation to generation, set and sung to music. Until written, the content evolves and changes over time, unlike a more traditional poem. Literary ballads are those composed and written formally.

Famous Ballads
Ballad of the Alamo
Ballad of the Green Berets