A narrative poem is one that tells a story. It follows a similar structure as that for a short story or novel. There is a beginning, a middle and an end, as well as the usual literary devices such as character and plot. A narrative poem can take the form of rhyming couplets, or it can go more in the direction of prose poetry, in that the rhyme scheme is flexible. There are many variations on the theme of the narrative poem.
The oldest known narrative poem is The Epic of Gilgamesh, a quasi-historical story about an Uruk (Sumerian) king who lived c. 2600BC. The earliest version of the poem itself was written on clay tablets in the 7th century BC. Around the same time, Homer of Greece composed the Iliad and Odyssey, still canon among today's classicists. Homer influenced later narrative poems like Beowulf (anonymous author, 8th-11th century AD); the Divine Comedy (Dante of Italy, 13 century AD); and the Rime of the Ancient Mariner (Samuel Taylor Coleridge of England, 1798AD). The style of narrative poetry is one of the most ancient forms of literature and yet continues to be used by poets.
A narrative poem tells a story in an entertaining way: with rhyme. As narrative poetry has its roots in ancient oral traditions, it is thought that the rhyme schemes were a mnemonic device that allowed performers to carry many stories inside them, before the advent of literacy. In the modern era, many musicians use narrative poetry to tell a story within the framework of a song, as in the case of many folk, country and hip hop artists.
Narrative poetry can be long or short form. The long forms are often a series of short story-poems meant to be told singly as an evening's entertainment (as in the case of Chaucer's Canterbury Tales or the heroic poems of Homer.) When a narrative poem does not contain a predictable rhyme scheme but does contain the various parts of poetry (alliteration, symbolism, and so forth), then it is a free-form narrative poem, or prose poem. The only limit to the types of narrative poems is the poet's imagination (and what the audience can appreciate).
A narrative poem usually contains a series of rhyming couplets (ABAB) or cinquains (ABABA) broken into stanzas (groups of lines), but the variations are endless. It can also contain any of the usual literary devices: alliteration, assonance, consonance, repetition, and so on. As the poem is a narration, it usually tells a story that has a beginning, middle and end, replete with character and plot development, climax and conclusion. The avant-garde, however, has done away with many of those rules, stretching the bounds between poem, story, and Dadaist tone poem (read: meaningful nonsense).
A narrative poem does not necessarily have to be linear or chronological. For example, the conclusion may be told at the beginning, and vice-versa. It can contain many divergent storylines, which may or may not weave their way back towards the main plot. The avant-garde and postmodern movements of the 20th century helped to redefine the narrative poem in this way.