Duvet Covers Discussions

Duvet Covers...questions?
I am interested in buying new bedding. i found some that i really like but it says it is a duvet cover....i 'm pretty sure that means its kind of like a giant pillowcase right? do i have to buy a duvet insert in order for this to be like a normal comforter? also, if i were to buy a duvet insert from another store would it fit the comforter if it is for the same size bed....like are all inserts the same standard size? anyone know where to buy any cheap inserts?? lots of questions but it would be really helpful if someone answered asap!!thanks! (Read answers)