King Arthur was a lengendary English king who is often written about romantically. That means great fantasy stories have been written about him. Records indicate that there may have been such a man in the early 6th century, but people disagree about if he was actually as just and formidable as literature presents him. Part of the reason for this is because so much of the literature was written much, much later... like in the 1800s.
His character is said to have created the Round Table. This gave him a united front with 12 specially selected knights who fought with him to keep Camelot, a perfect village, secure.
Even modern day renditions of Arthur's existence survive. The most recent film depicting Sean Connery as King Arthur was First Knight. Traces of several myths of the real King Arthur character dot that screenplay.
The attached enotes document gives great information, scroll down to a title called, "Decline, revival, and the modern legend" for most understood and relevant information.
There are perhaps two Arthurs.
First, there is the one of legend and Camelot, and the search for the Holy Grail and Excalibur, Merlin and the Lady of the Lake....and more.
The second is a warrior named Artorius (Lucius Artorius Castus). When Roman left Great Britian, a void of power was created and the island became vulnerable to invasion from the Saxons. Artorius stepped into to fill the void and unite the southern areas of England.
Thomas Mallory immortalized the story in his Morte d'Arthur and since then it has been retold by many. Disney's animated version, Sword and the Stone, as well as more realistic ones like Excalibur and First Knight continue to tell this story.
It is said that when England is in extreme peril, King Arthur will return and save England.