What is Doctor Who?

Well...Doctor Who is a long-running award-winning British science fiction television programme (and a 1996 television film) produced by the BBC. The series shows the adventures of a mysterious time-traveller known as "the Doctor", who explores time and space in his TARDIS time ship with his companions, solving problems and righting wrongs.
The programme is listed by the Guinness World Records as the longest-running science fiction television series in the world and is also a significant part of British popular culture. It has been recognised for its imaginative stories, creative low-budget special effects during its original run, and pioneering use of electronic music (originally produced by the BBC Radiophonic Workshop). In Britain and elsewhere, the show has become a cult television favourite and has influenced generations of British television professionals, many of whom grew up watching the series. It has received recognition from critics and the public as one of the finest British television programmes, including the BAFTA Award for Best Drama Series in 2006.
The programme originally ran from 1963 to 1989. A television film was made in 1996, and the programme was successfully relaunched in 2005, produced in-house by BBC Wales. (Some development money for the new series is contributed by the Canadian Broadcasting Corporation (CBC), which is credited as a co-producer, although they do not have creative input into the show.) Doctor Who has also spawned spin-offs in multiple media, including the current television series Torchwood and The Sarah Jane Adventures.
The relaunch of the programme has seen Christmas Day special episodes broadcast between series, the most recent being "The Runaway Bride". Series 3 of the relaunched programme, starring David Tennant as the Doctor and Freema Agyeman as his new companion Martha Jones, started on 31 March 2007 at 7pm on BBC One. A fourth series has been commissioned, which will air in 2008.

UPDATE: A current series which Stars Matt Smith as the 11th Doctor began airing in April 2010 and will continue to play the part in a new series slated to begin airing in 2011.