The Doctors fact file

The 10 incarnations of the Doctor from top left to bottom right.

The Doctor is an alien called a Time lord, the Time Lords come from the planet Gallifrey, Time lords can regenerate into new bodies 12 times if they are hurt badly or die.The Doctors age has been said to be around 900 however the Fourth Doctor stated to be 749 years old, the seventh Doctor said to be 953 years old, when the Tenth and Ninth Doctors speak of being 900 they refer to how long they have been travelling so seeing as he stated he started travelling when he was 236 years old then that would make the current incarnation around 1136 years old which would fit in with the sated age of the older incarnations. The Doctor usually travels with a companion/s to travel with him.
His arch enemies are the Daleks who fought a Time War with the Planet of Gallifrey(home of the Time Lords) and both sides were wiped out (it is implied that this was the Doctor's doing).
The different regenerations of the Doctor and their causes can be found below:

1. First Doctor (Hartnell): Apparently succumbed to old age, steadily growing weaker throughout and collapsing at the serial's end. Although the writer's intent was that this was due to the energy drain from the planet Mondas, this was not made clear in the transmitted story (The Tenth Planet).
   
2. Second Doctor (Troughton): A forced "change in appearance" and exile to Earth by the Time Lords in the closing moments of The War Games.
3. Third Doctor (Pertwee): radiation poisoning from the Great One's cave of crystals at the end of Planet of the Spiders.
4. Fourth Doctor (Baker, T): Fell from the Pharos Project radio telescope in Logopolis.
5. Fifth Doctor (Davison): Spectrox toxaemia, contracted near the start of The Caves of Androzani.
6. Sixth Doctor (Baker, C): Suffered unspecified injuries when the Rani attacked the TARDIS and caused it to crash land at the start of Time and the Rani.
7. Seventh Doctor (McCoy): Died in San Fransisco during exploratory heart surgery by a doctor unfamiliar with Time Lord physiology, after being hospitalised for non-life threatening gunshot wounds in the 1996 television movie.
8. Eighth Doctor (McGann): Not yet revealed but it was implied to be a result of theTime War.
9. Ninth Doctor (Eccleston): cellular degeneration caused by absorbing the energies of the space time vortex from Rose, which she in turn had absorbed through the heart of the TARDIS in The Parting of the Ways.
10. Tenth Doctor (Tennant):Current incarnation

The Doctor also is an enemy of the Master who is too a Time Lord but a evil one, the Doctor has battled him many time in many of his incarnations.