The 50th Anniversary prequel webisode (The Night of the Doctor) shines some light on this. (Spoilers!)
We saw the Eighth Doctor at some point during the Time War, where Time Lords are hated by other races (for the actions that lead to The End of Time, where they attempted to destroy all creation for their own ends). He stumbles upon the Sisterhood of Karn, which first appeared in the classic episode The Brain of Morbius. This isn’t the first time the Eighth Doctor has encountered them – they also meet in two Big Finish stories, Sisters of the Flame and Vengeance of Morbius.
The Sisterhood have detailed knowledge of Time Lord science, and (as explained in the episode), the ability to determine what a Time Lord regenerates into – personality, appearance, physical traits, and so on. This has precedence in the series – in the classic series episode Destiny of the Daleks, Romana seems to have fine-grained control over the bodies she regenerates into.
So the Doctor is offered the chance to control his regeneration, and knows that he can’t help people any more – being a Time Lord affiliates him with the war, and people turn away from him. He decides to try to put an end to it, and become a “warrior”. Once he’s regenerated, we see the reflection of John Hurt in some polished metal, establishing that his character really is another incarnation of the Doctor, who presumably fought in the Time War.
Indeed, this is verified in the end credits (he’s called The War Doctor). However, this might be a bit of a misnomer: in an interview for the latest issue of Doctor Who Magazine, Steven Moffat said:
He’s very specific, the John Hurt Doctor, that he doesn’t take the name of the Doctor. He doesn’t call himself that. He’s the same Time Lord, the same being as the Doctors either side of him, but he’s the one who says, ‘I’m not the Doctor.’
Presumably this is the effect of the change from “Doctor” to “Warrior”.
Obviously when the 50th Anniversary episode airs, we’ll get more details as to who he is and what he does.