Who is the Doctor?
You probably know the basics. The Doctor is an alien who appears human, but is in fact a Time Lord. This means they have two hearts and can cheat death by regenerating, a process which involves changing their body and personality.
The Doctor travels around in the TARDIS, a spaceship whose name stands for Time And Relative Dimensions In Space. This enables the Doctor to travel anywhere in the universe, including – you guessed it – across time.