Wow, you just asked the hard question.
What IS Doctor Who about?
Let's start with the main character, The Doctor. Not 'Doctor Who' just The Doctor. The Doctor is a Time Lord from the planet Gallifrey. He is also the LAST Time Lord. Their entire race was destroyed during the Last Great Time War against a race known as the Daleks, mutants housed within a mechanical outer shell who seek to destroy all non-Dalek life.
So then you have the Doctor, and he travels through time and space. He often brings along humans for the ride as companions.
Oh, and he's over 900 years old, the picture I posted is his eleventh incarnation. Time Lords are able to regenerate when mortally wounded, giving them a new personality and body while holding onto their memories.
There's a lot to explain honestly. The show's been around since 1963, and has had a lot of story piled into it.
The thing about the show is that what it's about largely depends on the episode in question. It's mostly science fiction, but when you have all of time and space to explore there's not much that's out of the question.