The Aeneid is an epic because is a long poem recounting the actions of men, gods, and heroes. Like the Greek epics on which it is modeled, the Aeneid uses the poetic meter of "dactylic hexameter." (You can read more about this in our section on "Writing Style.") As a writer of epic, Virgil was an innovator; before him, epic was thought of as purely telling stories of the distant past, and no one would have thought of making all the connections with recent political history that he does.
Virgil's epic is also influenced by the genre of "epyllion," or "little epic," which became popular among the sophisticated Greek Alexandrian poets of the third century B.C., and was then picked up in Rome by such poets as Catullus (who died when Virgil was in his 20s). This genre was always written in dactylic hexameter, but was on a shorter scale than true epic; it was usually written in a witty and learned style and dealt with the theme of love. Various scenes in the Aeneid, and especially the Dido episode in Book a, show the influence of the "epyllion" genre.