Doctor Who is a series that originally ran from 1963 to 1989. It came back in 1996 for a one off TV movie with paul McCan as the Doctor.
It returned next year for another run. The lead character is The Doctor. He is a Time Lord and travels through space and time in his Tardis. When his body wears out, or he gets hurt, he regenerates. Regeneration was the tool the production team used when the first Doctor had to leave the series over health issues. Regeneration works by rearranging the cells in his body, forming a new body with a new personality. The only thing that does not change is the Tardis, and his need to protect the innocent.
Recently Doctor Who has been very popular with Chrstopher Ecclestone, David Tennant and Matt Smith playing the Time Lord. The new series are different from the old ones in their high quality production design and CGI, the older servies had charming wobbly sets and monsters that looked more distressed than alien.
The most famous monsters / Aliens in the series are firmly in the collective psyche - Daleks, Cybermen, are easily recognized by adults and children alike.