Regeneration

From TARDIS Information System, the free Doctor Who reference database:

Regeneration is the process by which the Doctor and other Time Lords maintain their centuries-long lifespans. It is generally concieved of as a "genetic reshuffling" which a Time Lord may go through if his body is worn out by age or has suffered some life-threatening injury or infection. Time Lords may even regenerate at will.

Control over Regeneration
While the Doctor apparently has never had much choice in what the finished product looks like, others, like Romana, seem adept enough at the process to custom design their new appearances, as demonstrated in the opening scene of "Destiny of the Daleks," when she went through several bodies before finally deciding on a copy of Princess Astra. This is most likely because all the Doctor's regenerations have to date been involuntary, the result of mortal injuries or, in one case, having a regeneration forced upon him by the Time Lords prior to his exile on Earth at the end of "The War Games."

Development of the Concept
Regeneration was first introduced when the First Doctor (William Hartnell) changed into the Second Doctor (Patrick Troughton) in the closing moments of "The Tenth Planet." Tradition has it that Hartnell himself thought up the idea of the Doctor being able to change into a new body as a means of keeping the series going after his departure. In "Power of the Daleks" (the first story featuring the Second Doctor), the Doctor described himself as having been "renewed", and also said that the change was "part of the TARDIS."

The process was not actually referred to as "regeneration" until the end of "Planet of the Spiders," when the Third Doctor (Jon Pertwee) changed into the Fourth (Tom Baker). Prior to this, the Doctor was simply described as having "changed his appearance."