A ballad is a short narrative poem that tells a story. Ballads have one or two repeating lines, often with significant variation, like in Keats' "La Belle Dame sans Merci: A Ballad" that repeats the beginning of several different line sets. Ballads are usually written in the poet's local dialect or vernacular language, and they have much variation in stanza structure, meter, and rhyme scheme.
One popular American ballad is "The Ballad of Davy Crockett." The poem tells a brief biography of the famous pioneer from his birth in the Mountains of Tennessee to his involvement in the Creek Indian War, to his term in the US Congress, to his death at the Battle of the Alamo, and culminating with his posthumous legendary status as "The King of the Wild Frontier." Other popular American ballads exist about Jim Bridger, Kit Carson, and Vincent van Gogh.
Ballads need not be about real people. LeRoy Brown, Jim Walker, Big John, and Ringo, are all fictitious characters, but all have ballads about them. Typical themes of ballads are love, death, and the supernatural, like Coleridge's The Rime of the Ancient Mariner.
You may also have noticed that many of these ballads are also songs. Ballads were originally meant to be sung and many original folk ballads were accompanied by dances. Many songs are really just poems set to music. Poetry's tendency to have a rhythm lends itself to being put to music with a matching rhythm.