The Doctor comes from a planet called Gallifrey and those of his highly advanced species are known as Time Lords.
Well, the first thing to know is that the character is actually known as the Doctor (otherwise unnamed), as opposed to ever being called Doctor Who, which is simply the name of the long-running series.
So, just who is Doctor Who, you ask?
The Doctor rejected the boring oligarchy of his people, however, and went rogue with his stolen TARDIS (more on this magnificent machine later).