Doctor Who was originally planned as an educational series, it gave way to being a purely sci-fi television series. It is the longest and most successful science fiction series in television history, with a few breaks; it has been on since 1963. It is older than Star Trek - The Original Series by three years. It has spawned two cinematic movies so far, both of which are re-imagined versions of episodes. It stared out as multi-part 30 minute episode series but when it came back in 2005 after a break of 16 years, storylines tended to consist of single episodes of 45 minutes which made it easier to sell overseas, where the broadcasters can then turn them into hour long episodes.
The Doctor was a Timelord from the planet called Gallifrey who would go through a transformation/regeneration every so often when the actor left the series. The maximum number of regenerations he can do is twelve giving him thirteen bodies in which to regenerate into. He is currently on his thirteenth body with Peter Capaldi playing The Doctor. When Peter Capaldi leaves the series, the producers will come up with a way to extend the life of the doctor.
In the latter series, he told people he was the last Time-Lord but there was one other, The Master who would appear every now again to cause havoc. In the nineties special, the Master was consumed into the eye of the TARDIS. His escape is chronicled in a book which has not been made into an episode. The Time-Lords led by the Rassilon, Lord President of the Time-Lords (Timothy Dalton) appeared in the 2009 Christmas Special, End of Time. The Doctor sends them back with the assistance of the Master to where they came from. The Doctor had believed his had killed his fellow Time-Lords when they attempted to use `End of Time` device against the Daleks. The risks of the irreparable damage to the cosmos were too great for the Doctor to allow. The Doctor travelled the cosmos in a Police (Public Call) Box called the TARDIS (Time and Relative Dimension in Space). It wasn`t always a Police Box; a malfunction on-board prevented the machine from morphing into the background.
He rarely worked alone, there was always at least one person with him or he could call on U.N.I.T. (United Nations Intelligence Taskforce) when on Earth. The Deadly Assassin was one episode in which the Doctor had no companion to assist him. UNIT`s presence was most prevalent with the Third Doctor who had been exiled to Earth. The fourth doctor was assisted by a robot dog (K9) in a number of episodes starting from The Invisible Enemy up until his departure in the episode Warriors Gate. The robot made a re-appearance in a few episodes with David Tennant. K9 also appeared in the special `Five Doctors` episode. There was to be a spin-off series about K9 but it never got past the pilot episode unlike Sarah-Jane Adventures. Sarah-Jane Smith was a third and fourth doctor`s assistant who later investigated incidents in her own series along with children and K9. David Tennant guest starred as the Doctor in a couple of episodes which incidentally was the last time he played the character.
The Daleks are the fans favourite enemy of the Doctor, they are described as being moving pepper pots. The Daleks are from a planet called Skaro and they were once human but due to war and genetic experimentation by a Skaron scientist Davros, they now reside as creatures inside the pepper pots. People would joke that to escape a Dalek, you ran up a flight of stairs but that all changed in Remembrance of the Daleks where it was shown the Daleks could levitate.
The Daleks are the number one enemy of the Doctor, the number two is shared between the Cybermen and the Master. Cybermen were originally human like us coming from the planet Mondas, Earth`s twin planet. The inhabitents had managed to move the planet and in their introduction story (The Tenth Planet), their planet gets blown up. The Cybermen are humanoid robots who as well as having guns can also electro-paralyse their targets. The Cybermen guns have virtually disappeared from newer episodes. The Cybermen are controlled by someone known as the `Cyber Controller`. In the return series of the Doctor Who, Cybermen are from a parallel universe in the multiverse.
The Doctor is not the only Timelord left in the galaxy, the other is The Master. Whilst the Doctor travels the universe to help people, the Master is the complete opposite, he will travel to cause mayhem and see what power he can get for himself. Like the Doctor, he has the power to regenerate. it is said that he has used up all his regenerations.