Epic poems are classified as either primary folk epics or secondary epics, and are sometimes referred to as literary epics. Primary epics have no individually identifiable author and are derived from century's old oral traditions, whereas secondary epic poems are created by a single author whose style mimics folk epics. Some of the characteristics of epic poems shared by both types include broad geographic settings, an opening that states the poem's subject, and intense, heroic combat. Many epics also involve principal characters with god-like powers or superhuman capabilities who defend or extend their country's national interests during times of war while reflecting their culture's ideals.
Some of the characteristics of epic poems of the primary type include their creation by preliterate societies in which the story is orally transmitted to audiences and from one storyteller to the next. These oral epics are usually composed of brief episodes of a similar length and importance to the overall narrative, helping the storyteller to memorize the entire poem over time. The poem begins with the praeposito, a statement of the epic's theme or purpose. Literary epics influenced by Greek mythology begin with an invocation to the muses, the nine daughters of Zeus. Epic poetry often begins in the middle of the story's narrative arc, or en media res, depicting the hero under great stress, and then uses flashbacks to relate the beginning of the story.
Long lists of people, locations, and genealogies, known as enumeratio, are further characteristics of epic poems. These lengthy recitations were intended to both give the poems more universal appeal as well as to pay tribute to the listeners' ancestors. In many epics, the field of action and time frame is vast. For example, in Homer's Odyssey, the stories take place over the course of decades and encompass the entire known world of the Greeks. Frequent repetition of catch phrases, heroes who embody the culture's highest values, and divine intervention by gods are often found in epic poems.
The conventions established by Homer continued to influence the characteristics of epic poems long after his era. Often, the hero is engaged in a lengthy and arduous journey that includes a quest to gain valuable artifacts or prizes. The hero is also usually of noble birth and may include gods or demigods among his ancestors. While heroes often possess superhuman abilities, their invulnerability to harm is frequently compromised by a specific weakness, such as Achilles' heel. Phrases such as "resourceful Odysseus" are stock epithets that frequently recur throughout the epic and celebrate the hero's greatest qualities.