Ok, here we go
The Doctor is a Timelord, from the planet Gallifrey. He has a TARDIS, which can travel through all space and time. And there is a LOT of space of time.
He's very clever, and can usually figure his way out of things while saving a bunch of people, or other beings, but sometimes...it's too difficult. And he ends up being killed...well, humans would be killed.
Timelords can regenerate, instead of dying. They become different people, different looks, personalities, but the same memories. So that's why there are 11

They can be killed though, if they are killed before they have the chance to regenerate.
Also, The Doctor likes the company of humans, so he brings them along. but eventually they have to go, for various reasons.