Someone please explain duvet covers to me...

I never even heard of them til now! I found one that I like, as opposed to the comforter set I was originally going with. So I saw it in its package and it seemed awfully thing...do you have to order the filling that goes in it? What exactly is the filling?


Please excuse the completely STUPID question!

so I could put our ugly plaid comforter inside the duvet? No need to register for another comforter?