Have you by any chance read The Elegant Universe? Good book. It might help. (I suppose it might confuse you worse, as well. But I got a lot out of it.)

I hope im not wrong in remembering that M-Theory is String theory. Anyway, I dont fully understand the whole extra dimentions thing, but I read an explanation about an single extra dimention (other then x,y,z,t) that made a little sense. The explanation was given by one Michio Kaku and he used Carp as an example.

Suppose the carp in a pond had scientists. The carp can only move in two directions on top of the pond and they cant see down or up, so theirs is a 3d world. Now when it rains, you get ripples on the phase of the pond, and those ripples efect the carp. They have no idea how, the carp scientists only know 2D. So just like how the effects of the 3rd dimention are felt by the carp, some people say that light is the efect of the 5th dimention.

I know that dosen't explain very much at all. But just thought I'd throw it in there so people can comment on it as well.