Can anyone explain this to me?

Ok, so here it goes.

So far I've figured out that the latest Doctors are the 9th and 10th one, but I'm a bit confused. I've just started ever watching Doctor Who (The 9th Doctor one), and it has Rose in it. But I hear that Rose is also in the 10th Doctor on also, so did she come back as a different person (I.E. Returning actor), o did something happen to the Doctor o something (Idk really).

Just please explain this, it's really confusing.