The program depicts the adventures of the Doctor, a Time Lord, a time-travelling humanoid alien.
He explores the universe in his TARDIS, a sentient space-time vessel. Its exterior appears as a blue British police box, which was a common sight in Britain in 1963 when the series first aired; but its interior is far bigger than its exterior because it exists outside reality.
Along with a succession of companions, the Doctor faces a variety of foes while working to save civilizations, help ordinary people, and right wrongs.
Some consider Doctor Who (1963-1989, 2005-) to be (or have been) the longest continuously running genre television series in the world; others award that title to the Australian children's television show Mr. Squiggle (1959-1999). The longevity of Doctor Who is assisted by the Doctor having the ability to regenerate with (usually) a change of appearance.
Many fans of the series refer to the various manifestations of the Doctor by number (e.g., Four for the Doctor as portrayed by Tom Baker).
There was a Doctor Who movie in 1996. For a while the BBC, perhaps mindful of the Doctor having been assigned a limited number of regenerations, refused to count the lead actor, Paul McGann, as having played the Doctor. However, that has changed, and the 2013 Christmas Special has provided the Doctor with at least one additional regeneration.