The Doctor

The Doctor is a Time Lord from the planet Gallifrey. Being a Time Lord, gives him a rather interesting trick to cheat death. He can regenerate.
The process of regeneration is complicated and somewhat dangerous, but never the less, the Doctor has the ability to do it. When the Doctor is dieing, his cells regenerate, creating new features upon his face. Each Time Lord has a 12 regeneration cycle, allowing 13 lives. Once those lives are up, the Time Lord dies for ever.
There are ways to gain more regenerations though. One way, is to get another cycle from the planet Gallifrey, however this is impossible to do since the 9th Doctor explained that his planet was lost in the Time War. The second way, is to steal the rest of the lives from another Time Lord, as the Master attempted in the 1996 TV movie. However with the Time Lords supposedly dead and the Doctor the last of his people, this option also seems to be taken.
The Doctor has so far regenerated 10 times, using 11 of his 13 lives. The current Doctor is over 900 years old, and has seen, and done, many terrible and many great things.

Click the links below to find a small bio on that Doctor.

The First Doctor
The Second Doctor
The Third Doctor
The Fourth Doctor
The Fifth Doctor
The Sixth Doctor
The Seventh Doctor
The Eighth Doctor
The Ninth Doctor
The Tenth Doctor
The Eleventh Doctor