When you read Beowulf, unless you know Old English, you'll be reading it in translation, so you may not realize that it's actually a poem. In fact, it's written in alliterative verse, which is the kind of poetry the Anglo-Saxons used. Alliterative verse uses, you guessed it, a lot of alliteration—often three or even four words that begin with the same sound in each line.
It also has a strong pause, or caesura, in the middle of the line, and two strong stressed syllables on either side of the caesura. (So that's four stresses per line.) That may all sound pretty complicated, but actually it creates a really simple, easy-to-remember formula with a heavy rhythm to it.
We suggest you go check out an audio recording of Beowulf so that you can hear someone reciting a few lines in the original Old English. It's basically a "Dum Dum (pause) Dum Dum" sort of rhythm.
Why did the Anglo-Saxons use this heavily accented meter? Well, one persuasive theory is that most of their poetry was recited at feasts and other gatherings by bards who needed easy ways to remember it. This sing-songy rhythm made memorization easy—and it also made it easy to compose new poetry using established patterns.
Have you ever been listening to music on the radio and been able to complete the rhymes, even if you hadn't heard the song before, because they seemed obvious? That's because of the same kind of re-use of established patterns. Of course, sometimes we call these clichés, too, because they aren't very original. Anglo-Saxon poetry wasn't about originality, though—it was all about singing the praises of your hero.
Just. So. Epic.
Anyway, back to Beowulf. So Beowulf is a poem, but it's a very specific kind of poem—an epic. Like Homer's Iliad and Odyssey, or Virgil's Aeneid, or Dante's Inferno, Beowulf is a larger-than-life tale about heroic battles and journeys.
It takes place over many years—a little more than fifty years, in fact—and describes entire family trees of kings and lords in three different tribes. It deals with the deeds of man, but also with the plans of God and the relationship between God, man, and supernatural creatures. It travels between several different lands, sweeping across the sea, and even gestures at the wider context of all of Europe.
It's also very long—more than 3,000 lines survive, and there may once have been more. All these characteristics work together to give it a broad scope and truly make it, not just a poem, but an epic poem.