The Doctor is a time traveller and the central character of Doctor Who. Though he looks human, he is actually an extraterrestrial; a Time Lord from the planet Gallifrey.
We first see the Doctor in his ninth incarnation. The circumstances that led to his eighth regeneration are relatively obscure. The revived series follows a post-Time War Doctor, who somehow ended the war by wiping out the entire Time Lord and Dalek races. He decided to do so after he learned about Rassilon's (who was Lord President of the Time Lords during the War) plan, the so called 'Ultimate Sanction' . This included rupturing the time vortex, ripping it apart and destroying the entire Universe in the process, while the Time Lords would ascend to a higher level of existence to escape from the catastrophe they started and hence win the war.