Scop -- the Old English term for the poet, which means "the maker" (from the verb scieppan, "to shape, to make"). The poet shapes the poem, but he doesn't necessarily imagine or invent the story, which is likely to be traditional in content.

Alliteration -- the repetition of consonant sounds or vowel sounds in closely-placed words or syllables.  Since Anglo-Saxon poetry does not use rhyme, alliteration is a prime sound component in a line of Anglo-Saxon verse.

Caesura -- the space or pause in the middle of a line of Anglo-Saxon poetry that divides it into two half-lines or hemistichs. In Beowulf, each half-line contains two stressed syllables (and an indefinite number of unstressed syllables). Thus, each verse line of Beowulf contains four accented syllables and a varying number of unaccented syllables.

Kenning -- a figurative phrase that is a sort of simile, in which an item is described (often in terms of its function or action) without naming it. Examples from Beowulf: the sea is called "the swan-road" and "the whale-road"; a king is called "the ring-giver"; Hildeburh is called "weaver of peace." A modern kenning still in wide use is to call a large car a "gas-guzzler."

Litotes -- a deliberately steep understatement, often stated as a negative. Thus, the lake that is is the home of Grendel's mother is described thus: "That is no good place," which is one way of saying this lake is is a black, fearful, monster-infested pool of infamy. Similarly, one of the overwhelming praises of Beowulf is to say he "never cut down [killed]/ a comrade who was drunk," which means that Beowulf was a pretty trustworthy fellow in a pinch.

Sources:

Chickering, Howell D., Jr. Beowulf: A Dual-Language Edition. New York: Anchor-Doubleday, 1977.

Holman, Hugh. A Handbook to Literature. 3rd Edition. Indianapolis: Bobbs-Merrill, 1972.

Preminger, Alex, ed. Princeton Encyclopedia of Poetry and Poetics. Enlarged Edition. Princeton, New Jersey: Princeton University Press, 1974.