As a Gallifreyan, the Doctor looks human on the surface. However, there are differences. For example, they have two hearts. Also, if badly injured or dying, they can "regenerate". During the "regeneration", their body heals and renews itself. Then, they take on a different appearance and personality. They are the same person (sort of) with the same memories and knowledge. But, they look different and has a new personality. Because of this, the Doctor can be played by different actors. And in over fifty years, this is why the character has not actually "changed".