Beowulf is a heroic poem in Old English alliterative verse. At 3182 lines, it is far more substantial than any similar work in the language. The poem is untitled in the manuscript, but has been known as Beowulf since the early nineteenth century.
It is sometimes claimed to be the oldest surviving piece of text in what is an identifiable form of English, but this is highly disputable. The surviving manuscript dates to about 1000 AD. The poem itself is almost certainly older, but there is only circumstantial evidence for the date of its composition. Some experts suggest circa 800 AD on linguistic grounds. If that date is correct, there are older English texts.
The poem is a work of fiction, but it mentions in passing some people and events that were probably real, and probably from approx 450 to 600 AD in Denmark and southern Sweden. The story may have been taken to England by Danish migrants in some form (probably oral) and translated to English or rewritten at a later date in England.
The language used is called Old West Saxon, which is a dialect of Old English. It is one of the ancestor languages of modern English, but the language has changed so much over the years that most modern English speakers would not recognise it as their own language.
It is known only from a single manuscript, kept in the British Library. It is only partially legible after being fire damaged in 1731.
The story traces the life of a heroic king called Beowulf and his great battles with the monster Grendel, then Grendel's mother, and finally with a firebreathing Dragon in the battle which costs Beowulf his life. It is fundamentally a depiction of a pagan warrior society, in which the relationship between the leader, or king, and his thanes is of paramount importance. This relationship is defined in terms of provision and service: the thanes defend the interest of the king in return for material provisions: weapons, gold, food, drink. This society is also strongly defined in terms of kinship; if a relative is killed then it is the duty of surviving relatives to exact revenge upon his killer: this could be either with his own life or with weregild, a reparational payment. Moreover, this is a world governed by fate and destiny. The belief that fate controls him is a central factor in all of Beowulf's actions which occur in the poem.
There have been many translations of this poem. One of the best and also most recent is by the poet Seamus Heaney.
Here is a small sample including the first mention in the poem of Beowulf himself.
After each line is the translation by Francis Gummere to modern English (though the translation is still hard to follow). Gummere's translation is now also out of copyright, and can be had at Project Gutenberg.
 oretmecgas æfter æþelum frægn: ... asked of the heroes their home and kin
 "Hwanon ferigeað ge fætte scyldas, "Whence, now, bear ye burnished shields,
 græge syrcan ond grimhelmas, harness gray and helmets grim,
 heresceafta heap? Ic eom Hroðgares spears in multitude? Messenger, I, Hrothgar's
 ar ond ombiht. Ne seah ic elþeodige herald! Heroes so many ne'er met I
 þus manige men modiglicran, as strangers of mood so strong.
 Wen ic þæt ge for wlenco, nalles for wræcsiðum, 'Tis plain that for prowess, not plunged into exile,
 ac for higeþrymmum Hroðgar sohton." for high-hearted valor, Hrothgar ye seek!"
 Him þa ellenrof andswarode, Him the sturdy-in-war bespake with words,
 wlanc Wedera leod, word æfter spræc, proud earl of the Weders answer made,
 heard under helme: "We synt Higelaces hardy 'neath helmet: -- "Hygelac's, we,
 beodgeneatas; Beowulf is min nama. fellows at board; I am Beowulf named.
 Wille ic asecgan sunu Healfdenes, I am seeking to say to the son of Healfdene
 mærum þeodne, min ærende, this mission of mine, to thy master-lord,
 aldre þinum, gif he us geunnan wile the doughty prince, if he deign at all
 þæt we hine swa godne gretan moton." grace that we greet him, the good one, now."
 Wulfgar maþelode (þæt wæs Wendla leod; Wulfgar spake, the Wendles' chieftain,
 his modsefa manegum gecyðed, whose might of mind to many was known,
 wig ond wisdom): "Ic þæs wine Deniga, his courage and counsel: "The king of Danes,
 frean Scildinga, frinan wille, the Scyldings' friend, I fain will tell,
 beaga bryttan, swa þu bena eart, the Breaker-of-Rings, as the boon thou askest,
 þeoden mærne, ymb þinne sið, the famed prince, of thy faring hither,
 ond þe þa ondsware ædre gecyðan and, swiftly after, such answer bring
 ðe me se goda agifan þenceð."
as the doughty monarch may deign to give."