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The Doctor is a Time Lord, an extraterrestrial from the planet Gallifrey, who travels through time and space in an internally vast time machine called the TARDIS (Time And Relative Dimension(s) InSpace) which appears relatively small when seen from the outside.
The Doctor explores the universe at random, using his extensive knowledge of science, technology and history to avert whatever crisishe encounters. The imprecise nature of his travels is initially attributed to the age and unreliability of the TARDIS's navigation system. However, the 1969 serial The War Games reveals that the Doctoractually stole the TARDIS, and subsequent stories such as "Planet of the Dead", "The Big Bang" and "The Doctor's Wife" have incorporated this. Additionally, it has been mentioned that the TARDIS ismeant to be piloted by six Timelords, rather than just one. He was presumably unfamiliar with its systems but was able to operate it correctly until his exile when the Time Lords wiped it from hismemory. The Doctor initially had the manual for operating the TARDIS but destroyed it (by throwing it into a supernova) because he disagreed with it. After his trial and exile to twentieth century Earth,the Doctor still visits other planets on missions from the Time Lords who pilot the TARDIS to precise locations for him.
After his exile is lifted, the Doctor returns to his travels and demonstratesthe ability to reach a destination of his own choosing more often than not. In the 2011 episode "The Doctor's Wife", the Doctor tells the TARDIS (whose matrix, or soul, was temporarily transferred tothe character Idris) that she has never been very reliable in taking him where he wanted to go. The TARDIS explains that she always took the Doctor where he needed to be. In "Journey's End", theDoctor states that the reason for the previous bumpy navigation was that the TARDIS is meant to have six pilots, but in "The Time of Angels", River Song demonstrates superior piloting skills and says the...