As a Gallifreyan the Doctor looks like a human on the surface, but there are differences. One well-known difference is that he has two hearts. As a Time Lord the Doctor can also "regenerate", if his body is badly hurt and he dies. During the "regeneration", the body is healed and renewed, which includes a different appearance and personality. The regenerated person is the same person, with the same memories and basic personality traits; but the person also looks different and a changed personality. Because of this the Doctor could be played by different actors. This made it possible for the series to be filmed over a time of nearly fifty years without having to actually "change" the character.