Doctor Who is a British television series about a Time Lord. This is an alien who looks totally human, but possesses two hearts and has the ability to regenerate into a different body - and therefore a different actor - when he is on the edge of death. There used to be a lot of Time Lords, but according to the current run of the television series, the Doctor is the last survivor of a terrible battle with the Daleks, a murderous race of robot-aliens. He bounces around time and space, accompanied by one or two 'companions' at a time, in an unexpectedly roomy blue telephone box called the TARDIS, averting disasters and saving the galaxy. I guess the telephone box was deemed nicely inconspicuous by the misguided soul who designed it.
The regeneration aspect is a handy one for the producers behind the series. If the actor who plays the Doctor wants to bow out, the character can be mortally wounded and regenerate. This actually happened after the first series in the new incarnation, when Christopher Eccleston decided to leave, causing 'his' Doctor to morph into David Tennant. (Tennant is still going strong and wildly popular, so don't expect him to change again soon if the producers get their way.) What adds to the fun is that the Doctor's personality is allowed to change slightly to suit the new actor. While in other series it would be a headache to have the lead leave, for Doctor Who it just allows a new dynamic and new stories.
Doctor Who was off the air for a long time, originally running from 1963 until 1989. It was revived in 2005 by Russell T. Davies of Queer as Folk fame to success beyond all expectations. The fan base of the sci-fi series rivals that of Star Trek in their fanaticism, so it was quite an achievement that Russell managed to both placate them and hook a new generation of viewers. I think he played fast and loose with some of the continuity of the previous series, but I can't be sure as there is too much material in the form of novels, radio plays, and episodes for me to wade through.