(Get relaxed because there is a ton of explaining to do. Plus i was really hoping to get this message because i love to tell this.)
Okay so basiclly the doctor is an alien actually to be more specific a time lord form the planet Gallifrey and his race is all wiped out because he destroyed all of time lords in the time war, no need to get into details, and so far he has had 12 technically 13 reincarnations. So 13 people played him. The actors are
William Hartnell ,
Patrick Troughton, Jon Pertwee, Tom Baker, Peter Davison, Colin Baker, Sylvester McCoy, Paul McGann, Christopher Eccleston, David Tennant (your doctor), Matt Smith, Peter Capaldi, and John Hurt played the doctor who fought in the Time War and he isn't considered a doctor until The Day of the Doctor which was a special that aired in November, anyway the reason he changes ever two seasons or so is because every time he changes he dies. You see, the doctor is over 1,000 years old and that is because his body dies, so every living cell in him dies but his body forces them to change, so therefore he himself must change. It's actually called 'Regeneration' and he regenerates into someone else. And in the, i think, 50 minutes in his regeneration he can rebuild body parts, so if you chopped off his hand then he could re-grow it (And that has happened) So the doctor you watched is the tenth doctor and he is a really cute dude......sorry for no periods.......OH! And he once he regenerates he is still in his old clothing and he will change them but he never changes out of them until he regenerates again. David Tennant is a very funny guy. Plus the only thing the doctor has as a weapon is a sonic screwdriver. Whew! Totally Nerded out all over this message.