Doctor Who is a British science-fiction TV show produced by the BBC that narrates the adventures of the Doctor, a Time Lord (an alien humanoid with two hearts and a very distinct sense of humor) who explores the universe in a spaceship called the TARDIS (Time And RelativeDimension In Space). At first sight, the TARDIS is nothing more than blue British police box (very common in Britain in 1963 when the first series was aired), but when the door opens a big surprise is revealed: its interior is bigger than its exterior and you can say that it’s virtually infinite inside.
Because the Time Lords’ alien technology, the TARDIS can travel in time and space and, among other abilities, can camouflage itself becoming invisible. So The Doctor, accompanied by his invaluable companions, travels through different galaxies and different eras to help civilizations and fight a large number of evil villains, also keeping his soft spot for the Earth, of course.
The series can be divided in two different production periods, the first one usually called the “classic series“, from 1963 to 1989, and the revival, some times called the “new series“, from 2005 to nowadays.
Over the years the Doctor was interpreted by twelve different actors, which means, twelve different faces to the Doctor. The changing of the character’s face and personality is explained in the series by the need of regeneration in a new incarnation whenever the Doctor suffers a serious injury that can lead him to death. Even with different faces and slightly different personalities, each incarnation of the Doctor is a facet of this centenary alien being of complex personality, within the same story line. This way of accommodating the casting changes that invariably occur in a long series like this can be considered one of secrets for its long success.
(*) Sources: Wikipedia and Doctor Who at BBC.