The King Arthur described in the collection of stories and poems called the Arthurian Legend never really lived.
During the 6th century, after the Romans had left Britain, a man named Arthur was the general who led the people of Britain against the Saxon invaders from Germany in the early 500s.
Arthur achieved a number of great victories, but he was never a king, and all the stories about the Knights of the Round Table, Camelot, the Search for the Holy Grail (the cup Jesus used at the Last Supper), Queen Guenevere, Sir Lancelot and the other Arthurian heroes grew up centuries later.
They were based partly on the story of the real Arthur and partly on other legends and myths.