Beowulf is one of the oldest surviving epic poems in what is identifiable as an early form of the English language. In the poem, Beowulf, a hero of a Germanic tribe from southern Sweden called the Geats, travels to Denmark to help defeat a monster named Grendel. This poem, about Danish and Swedish kings and heroes, was preserved in England because the English people are descendants of Germanic tribes called the Angles, Saxons, and Jutes. Jutes and northern Saxon tribes came from what is now southern Denmark and northern Germany. Thus, Beowulf tells a story about the old days in their homeland.
The poem is a work of fiction, but it mentions a historic event, the raid by king Hygelac into Frisia, ca 516. Several of the personalities of Beowulf (e.g., Hrothgar, Hrothulf and Ohthere), clans (e.g. Scyldings, Scylfings and Wulfings) and some of the events (e.g. the Battle on the Ice) also appear in early Scandinavian sources, such as the Prose Edda, Gesta Danorum, the legendary sagas, etc. In these sources, especially the Hrólf Kraki tales deal with the same set of people in Denmark and Sweden (see Origins for Beowulf and Hrólf Kraki).
Consequently, many people and events depicted in the epic were probably real, dating from between 450 and 600 in Denmark and southern Sweden (Geats and Swedes). As far as Sweden is concerned, this dating has been confirmed by archaeological excavations of the barrows indicated by Snorri Sturluson and by Swedish tradition as the graves of Eadgils and Ohthere in Uppland. Like the Finnsburg Fragment and several shorter surviving poems, Beowulf has consequently been used as a source of information about Scandinavian personalities such as Eadgils and Hygelac, and about continental Germanic personalities such as Offa, king of the continental Angles.
Beowulf was the greatest hero who ever lived!