A long narrative poem is called an epic. These types of poems were used to write such well known pieces as the Iliad and Beowulf. They are normally written in hexameter style.
The hexameter style of writing for an epic poem is hard to write in English. This is the reason why epic poems that are written in English are written in a different type of style. There are multiple styles of English epic poems, but the most common are pentameter, blank verse and alliterative. Most epics share common characteristics, such as battles, heroes or vast settings. The writers of epics often stated their theme right at the beginning so that the reader knew what was to come and to set the scene.