She is an alien, a "Time Lord" from the planet Gallifrey. The Doctor travels in his space-time-ship, the TARDIS, (Time And Relative Dimensions In Space) with which she can go anywhere in time and space. Because the TARDIS' "chameleon circuit" is broken, it looks like an old British police-call box permanently. The Doctor often takes other people with him, who are usually called "companions" or "assistants". They are always 21st century humans in the modern series, however the classic series often had human-looking aliens or people from the past or future. The most recent companions are Yaz and Dan.