When doctor who first started he was said to only have a few regenerations. This is when he became a new person and also so they could keep the show running. In a regeneration the doctor completely changes his looks and personality so each one is very different. As the show has been running for so long the doctor has regenerated 12 times now. Matt Smith being the 12th doctor and Peter Capaldi is the 13th and current doctor. Several months ago Matt Smith announced he was leaving Doctor Who and so Peter was chosen to replace him and at the end of the last series the doctor regenerated into this one. Which is why Peter Capaldi is the current doctor.

It started back in 1966 when the first actor to play the Doctor Was too ill to continue so the producers/writers came up with the idea that since The Doctor Was an Alien he could Regenerate into a different body/face and personality and so they could hire a New Actor-

- so now when an Actor wants to Move on to other Roles they have the Character Regenerate

( Doing Doctor Who can take 9-10 months including Publicity so it takes up ones professional life)

Yes LadyMerton is right in saying the actor grew ill and they needed to replace. With him being an alien they could legitimately change the actor and come up with an explanation. Troughton, the replacement actor was originally going to play the part more similarly to Hartnell before him but they decided to just change the characters personality along with his face, which was a clever move because it gave them great flexibility with the character. Originally it wasn't called regeneration, it was called renewal. Troughton gave up the role after 3 years because he didn't want to be typecast to the character.

All that is correct, so can't add anything. So I will beat LadyMerton on one thing, which can never be outdone only equalled. Been a fan of Doctor Who for 51 years. Since the day it started. lol