The Doctor
"There was a war. A Time War. The Last Great Time War. My people fought a race called the Daleks, for the sake of all creation. And they lost. We lost. Everyone lost. They're all gone now... my family... my friends... Even that sky. Oh, you should have seen it! That old planet... The second sun would rise in the south, and the mountains would shine. The leaves on the trees were silver, when they caught the light, every morning it looked like a forest on fire. When the autumn came, a breeze would blow through the branches, like a song..."
The Tenth Doctor, Gridlock

The Doctor is  a 903 year old Time Lord from the planet Gallifrey.  Unlike other Time Lords, his mission is to help people on other planets and times, defending them from villains like the Daleks, Cybermen and the Master.    He can regenerate into a new form when fatally injured, has two hearts, and has some super-human abilities.  He is in his tenth incarnation.  The Doctor fought in the Time War, where the Time Lords and Daleks were both destroyed, along with Gallifrey.  He often travels with companions who help him solve problems he encounters. 

 

The Doctor's adventures have been chronicled in the television series from the BBC 1963-1989, 1996 and 2005 to the present, as well as original novels, novelizations of episodes, audio adventures, plays, films, and comic books.  He has been portrayed by ten actors on television: William Hartnell, Patrick Troughton, Jon Pertwee, Tom Baker, Peter Davison, Colin Baker, Sylvester McCoy, Paul McGann, Christopher Eccleston, and David Tennant.  Other Doctors who are not considered part of the canonical series are Peter Cushing in two feature films: Dr. Who and the Daleks and Daleks- Invasion Earth 2150 AD and Richard E. Grant in the 40th anniversary animated webcast Scream of the Shalka on bbc.co.uk.  While the television series produced by the BBC is canon, the other stories' canonicity is unclear.  Some of the books and audio plays have been the inspiration for television episodes in the new series, like Dalek, Rise of the Cybermen/The Age of SteelHuman Nature/The Family of Blood, and BlinkThe Doctor Who Reference Guide has a listing of where all the stories in various media fit in to the television continuity, with a detailed description of each adventure.