The Doctor is a humanoid alien who time-travels to help defeat evil, help people, and correct wrongs in the universe. His method of transit is a space ship named TARDIS (short for Time and Relative Dimension in Space). It looks very much like an old-fashion British police box.
The Doctor has taken the form of many different humans over the years, largely in part to his ability to regenerate as a new entity. Each time the Doctor is injured, to a point that would otherwise have been fatal to humans, he takes on this new body and personality. As a Time Lord, he can only regenerate 12 times, totaling 13 forms.
Each Doctor has his own set of quirks and style including the Eleventh Doctor’s bow tie, the Tenth Doctor’s brown jacket, the Eighth Doctor’s cravat, the The Seventh Doctor’s umbrella, the Sixth Doctor’s rainbow coat, and The Fourth Doctor’s scarf.
Although written out of some of the Doctor’s history, the sonic screwdriver remains an important element in the Doctor’s arsenal of tools.