[spoiler]The Doctor is the last of his kind, a species know as the Timelords from the planet of Galifrey. These people were advanced enough to make machine that could travel through time and space. The Timelords eventually became engaged in conflicts called The Great Time Wars with another alien species, called the Daleks. The Doctor ended the last of The Time Wars and saved all life in the galaxy, at the cost of killing both his people and the Daleks. He now travels the galaxy with companions, saving lives in non violent ways, to atone for his actions. Like the rest of his race, he can regenerate into a new body and personality when mortally wounded. [/spoiler]