At this point I think it wise to explain the Doctor's origins. The Doctor is a renegade and was even put on trial twice by his nearly omnipotent peers who refuse to use their knowledge and power to fight the good fight. As a result of the first trial's verdict, the Doctor lost his life. No not really. Time Lords have the ability to regenerate twelve times.
Regeneration refers to a total cellular make-over.
There have been seven Doctor. The same mind in the same body but with different forms. The first Doctor was played by William Hartnell. When Mr. Hartnell became to ill to continue, Pat Troughton was called in as a replacement. Mr. Troughton was several feet shorter and several years younger than Mr. Hartnell.