There were a few scattered references to him in early medieval texts from around the time when he would have been alive (one poem mentions a warrior 'who was not Arthur'), but they don't tell us any details about who he was or what he did. The first more detailed accouts we have about his life come from a ninth century Welsh chronicle, by which time he had become a useful legendary figure around whom the chronicle's countrymen could rally in their wars against the neighboring Anglo-Saxon kingdoms - a mythical rallying cry for their contemporary conflicts. It's clear that these accounts are only pseudo historical at best, however, and as time went on Arthur became increasingly legendary and his historical reality becomes impossible to trace.

He was most probably a real person, but more than that is impossible to say.

I wrote a slightly longer answer about this last year.

The best book dealing with this question is Worlds of Arthur, by Guy Halsall (2013). It's written for a non-specialist, but has an excellent examination of the historical sources, discusses and critiques other attempts to find the historical Arthur (which have all failed for various reasons), and offers a good appraisal of what we can and can't know about the fifth or early sixth century when Arthur would have been alive. It's inexpensive, and well worth the read if you want to learn more about this question.