The Doctor is an alien from the planet Gallifrey. His people, the Time Lords, had fought a horrible war with the Daleks - the arch nemesis of the Time Lords. To quote the Doctor,
There was a war and we lost
Now, the only surviving Time Lord, The Doctor travels around the universe doing what he does best: Fighting aliens and saving people. His favorite planet is earth. Though sometimes he thinks humans are very stupid, he does have a fondness for them that has him coming back to earth time and time again.
Because of the war and his part in it, The Doctor is very angry and somewhat callous to the world. Using his TARDIS, he flies through space and time always on the run. He is a lonely traveler, the last of his kind.