Written in Anglo-Saxon, or Old English, sometime between the 8th and 11th centuries, Beowulf is an epic poem that reflects the early medieval warrior culture of Europe. Although scholars still debate some of the particulars, essentially Beowulf is the oldest surviving significant piece of literature in English. However, it's not the very oldest surviving thing written in English – that, if you wondered, is probably "Caedmon's Hymn," although scholars like to argue about that, too. Beowulf is also not in any kind of English that the average, modern English-speaker could recognize or read, since Anglo-Saxon was the language spoken in Britain before the Norman Conquest in 1066 – that is, before the extensive influence of French on the language we speak today. Still, Beowulf has come to be recognized as the foundational epic of English and British culture, in much the same way that the Iliad and the Odyssey are the foundational epics for ancient Greece.
Although it was written and recited in Britain, Beowulf is about characters in Scandinavia – Danish and Swedish warriors who battle fabulous monsters as well as each other. Why? Because the early Anglo-Saxons were the descendants of Germanic and Scandinavian tribes that invaded Britain beginning in the 5th century. As a result, there was a lot of shared cultural background between the Anglo-Saxons and Scandinavians, and the Anglo-Saxons looked back to their relatives across the sea when they wanted to tell stories about their own past. So Beowulf isn't just a story told in one place (Britain) about another place (Scandinavia) – it's also told in one time about an earlier time. We know this because the culture in which the poem was recited, 8th to 11th-century Britain, had already been Christianized, but the Scandinavian culture that the poem describes is still pagan. Thus the poem is a way for the Anglo-Saxons to describe their own past – their ancestors who came from over the sea centuries before.
There are several different theories about the composition of Beowulf, but the "original" would probably have been part of the oral culture of Anglo-Saxon Britain, a long epic or a series of short tales recited by bards at feasts and other gatherings. At some point between the 8th and 11th centuries, the epic was written down in a manuscript called the Nowell Codex, but it remained unpublished until 1815. In modern publications, the poem has been given the title Beowulf, although in the original manuscript it is untitled.
Since its publication and translation two centuries ago, Beowulf has captured the attention of scholars and audiences alike, becoming a keystone of English literary studies as well as the basis of several popular film and TV adaptations. J.R.R. Tolkien, a professor of languages at Oxford in the early 20th century, was especially interested in Beowulf. In 1936 Tolkien gave a lecture entitled "Beowulf: The Monsters and the Critics," which was highly influential in establishing the epic as an important part of literary history. Tolkien also used many elements from Beowulf as inspiration for his famous "Lord of the Rings" trilogy. But whether it is interpreted by critics or enjoyed as an adventure story, Beowulf has become one of the most important pieces of literature in English.
Come on, do we really have to tell you why you should care about Beowulf? Beowulf, a great and glorious hero arrives from over the sea, clad in a shirt of shining mail, ready to do barehanded battle with a demonic monster.
If that leaves you wanting more, Beowulf is ready to deliver. Once the demonic monster bites the dust, his bigger, badder, even more demonic mom arrives to avenge her son's death. But that's still not the climax. Just in case anyone doubted Beowulf's prowess at this point, a dragon shows up to test him to the limit. This isn't dry-as-dust literature that you fall asleep over; it's the kind of thing you pay ten bucks to see while eating popcorn! (Although the recent Beowulf movie goes just a little bit off-script.)
OK, if you're still not hooked, try this: Beowulf is the oldest major work of literature in English. In fact, it's in such old English (technical name: "Old English") that it seems like a foreign language to us today, because our words have changed so much since it was written. It's a glimpse of an ancient Anglo-Saxon and Scandinavian culture. But this history lesson isn't just names, dates, and agricultural innovations. Instead, it's gleaming golden armor, straining sinews, and wild drunken parties that go all night because everyone would rather tell stories about past glorious victories than think about the fact that they'll probably die horribly tomorrow. It's a brutal world, but one that offers the possibility of fame – and maybe even fortune, if you're lucky.
In short, it's dragons and demons and heroes, and it'll get you some street cred with your teacher for having read the first recorded epic poem written in some form of English.