British Television barely makes the rounds in the United States, so it may come to no surprise you don't know what Doctor Who is. The short of it is that it's Britain's version of Star Trek. It's just as big and as important to the genre of science fiction. Unlike Trek however, the format allows for season-to-season experimentation.
The series tells the story of a man known as The Doctor, who is a lord of time and space. While he carries the title proudly, he engages in the vastness of the universe rather than act as a mere spectator. To do so, he steals a time machine that the Timelords (as they are called) use to travel around. The uniqueness of this particular machine is that it can change into anything on the outside. For the sake of keeping a budget on the show even in new series, the machine is disguised as a police box (a phone booth in which you can call the police) on the outside. When The Doctor heads inside the police box, the inside is massively bigger. In the series early days, this allowed the producers a chance to combine live and in-studio filming to prevent having the building of a massive set.
But the key to the series' success is largely the character of the Doctor. He is a lexicon of humanity, portraying every facet of human emotion through each of his lives throughout the series. He could be cold, warm, erratic, violent, deceitful and funny; sometimes all at once, and sometimes individually in each life. Because the character is this lexicon of personalities, choosing the actor can be a challenge in itself.
Russell T. Davies took on this challenge by hiring Christopher Eccelston, a man known for his dark and intense performances. Unfortunately, Eccelston is well known as an actor who bounces from part to part and doesn't usually stick around. This suited Davies fine, as the purpose was to jumpstart the program back into the limelight. As soon as the ratings got good enough and the interest returned, he quickly signed a deal for three more years and David Tennant, a man known for his light approach to his characters, was hired. How this would take place would be new to novice viewers of the series, and familiar to past generations.