King Arthur by most sources was a romantic, legendary, fable created during the Dark Ages of England. He was popularized in Geoffery Monmouths History of the Kings of Britain in this tale, the rise and fall of Arthur is told. Arthur came to prominence under Uther and eventually rose to take command of the British armies to repel a threat from the saxons. During this tale, Arthur slayed a dragon and of course was betrayed by Mordred.
The legend of Arthur was expanded upon in the follow centuries. Elements such as Camelot, Merlin, and the Knights of the Round Table were added. The stories of Perceval, Galahad, and Lancelot were thrown into the mythos, and by the 21st century a longstanding legend of Arthur and his fabled knights of Camelot had been established. This myth has been shown in movies such as Excalibur, King Arthur, and First Knight to name a few.
But, what is the real story of Arthur? Or was it always a myth? There is likely some kind of historical figure that was the genesis of Monmouth's legend. A few Historians believe that Arthur was a historical figure that existed between the 5th century and the 6th century. During these centuries Arthur fought to repel saxon invadors. This explanation is explored in King Arthur the movie. In this movie, Arthur was a roman knight who fought on after the Roman legions withdrew in the 4th century. He united the scattered tribes of Britain and Wales to fight Saxon invadors from Germania. This is a likley explanation and has some historical context. Ancient historians such as Nennius provide some detail to Arthur and his campaign. However, most historians exclude Arthur from their history of post-Roman Britain because of the lack of historical evidence. Most evidence comes from one account that might have been added as late as the 9th century, based on oral tradition.
I do not know if Arthur was real or not. There is little historical evidence to support such a claim, and an overwelming amount of evidence to exclude him from history. However, he should be remembered as a legend, and even if he never existed he has become one of the most popular post-roman legends of Britain.