Doctor Who is a science fiction television program invented by the BBC. The series stars a man from another planet who can travel anywhere in space and time, and who usually brings along human companions with him. The man is known simply as "the Doctor," and his true name is never known. (His last name is not "Who." The title of the show is a question: "Doctor who?")
The Doctor is from a planet called Gallifrey, whose race is called Time Lords. He travels in a spaceship called "Time and Relative Dimensions in Space," or the TARDIS for short. Through a science unknown to man, the TARDIS is bigger on the inside than it is on the outside. The TARDIS is supposed to change into a different shape to blend into its surroundings (it would appear as a pyramid in ancient Egypt, or as a tree in the forest), but after stopping in 1960s England and assuming the shape of a British police telephone box, it got stuck in that shape and continues to look like a telephone box wherever it goes.
A central conceit of the series is that the Doctor, like all members of his race, can regenerate into a different body is the old one gets fatally injured. As such, a new actor can take on the role every few years, regardless of the differences in physical appearances to any of his predecessors. To date, there have been 12 regular actors who have portrayed the Doctor on a weekly basis (not counting one-off appearances, non-canon appearances, or parodies).
The series initially ran on the BBC from 1963 to 1989 in half-hour installments, with most storylines serialized over a number of weeks. The series relaunched in 2005 as a weekly one-hour series, with many episodes being standalone, but with occasional two-part stories and often having an overarching plot over the course of a season or two. As of this writing (Apr. 19, 2015), the series is still very popular and has been renewed by the BBC through the year 2020.