He was the mythical, legendary King of Camelot and the Round Table.
http://en.wikipedia.org/wiki/King_Arthur
Arthur led during the late 5th and early 6th centuries, according to medieval histories and folk tales and legends. He is the boy who drew the sword from the stone to become king.
From Wikipedia:
The legendary Arthur developed as a figure of international interest largely through the popularity of Geoffrey of Monmouth's fanciful and imaginative 12th-century Historia Regum Britanniae (History of the Kings of Britain). In some Welsh and Breton tales and poems that date from before this work, Arthur appears either as a great warrior defending Britain from human and supernatural enemies or as a magical figure of folklore, sometimes associated with the Welsh Otherworld, Annwn. How much of Geoffrey's Historia (completed in 1138) was adapted from such earlier sources, rather than invented by Geoffrey himself, is unknown.
There is very little historical basis for Arthur's existence:
http://en.wikipedia.org/wiki/Historical_basis_for_King_Arthur
The historical basis of King Arthur is a source of considerable debate among historians. Due to the poverty of British records in the period 450-550 CE, historian Thomas Charles-Edwards noted that "at this stage of the enquiry, one can only say that there may well have been an historical Arthur [but …] the historian can as yet say nothing of value about him". Historian David Dumville summed up his position by saying, "I think we can dispose of him [Arthur] quite briefly. He owes his place in our history books to a 'no smoke without fire' school of thought ... The fact of the matter is that there is no historical evidence about Arthur; we must reject him from our histories and, above all, from the titles of our books."
Some have suggested that Arthur was a mythological or folklore figure, that other mythological figures also may have become historicised: one suggestion is that Hengest and Horsa were originally Kentish totemic horse-gods, ascribed a historical role by Bede. There is, however, no more early trace of this fictional Arthur than there is of a historical one.
Arthur appears in a historical context as a British soldier (miles in the original Latin) fighting alongside British kings against the invading Saxons in a Latin text of the 9th century, more than three centuries after his supposed floruit in 5th-6th century Sub-Roman Britain. The legendary king of the Britons of Arthurian legend develops from the 12th century after Geoffrey of Monmouth's influential Historia Regum Britanniae.
Although the themes, events and characters of the Arthurian legend varied widely from text to text, and there is no one canonical version, Geoffrey's version of events often served as the starting point for later stories. Geoffrey depicted Arthur as a king of Britain who defeated the Saxons and established an empire over Britain, Ireland, Iceland, Norway and Gaul. Many elements and incidents that are now an integral part of the Arthurian story appear in Geoffrey's Historia, including Arthur's father Uther Pendragon, the wizard Merlin, Arthur's wife Guinevere, the sword Excalibur, Arthur's conception at Tintagel, his final battle against Mordred at Camlann and final rest in Avalon. The 12th-century French writer Chrétien de Troyes, who added Lancelot and the Holy Grail to the story, began the genre of Arthurian romance that became a significant strand of medieval literature. In these French stories, the narrative focus often shifts from King Arthur himself to other characters, such as various Knights of the Round Table. Arthurian literature thrived during the Middle Ages but waned in the centuries that followed until it experienced a major resurgence in the 19th century. In the 21st century, the legend lives on, not only in literature but also in adaptations for theatre, film, television, comics and other media.