There is no difference between a quilt and a duvet.They are both the same things
really a duvet is a posh word for a quilt,they both go inside your quilt or duvet cover.
You will find differences in sizes though,some large department store sell quilts and
duvets at slightly different sizes,this is because the sizes of the covers are slightly
different sizes too.This is so that you have to buy the quilt/duvet cover from the store
which are normally more expensive.
This does not happen very often now a days but it is worth asking the question.
the normal sizes of Quilts/Duvets are as follows.
Single Size 54"x 78"
Double Size 78"x 78"
King Size 86"x 98"
Super King Size 102"x 86"