I assume this is a joke, but just in case you really don't know, he's a time-traveling alien from the planet Gallifrey. It's a TV series by BBC that started in 1963 and is still going strong, although it didn't become very popular until 2005. If you start watching it, that is where you should begin - the 2005 episodes. Then, go back and watch the older ones after you decide you like it.