Stuffing is where you fill the anal cavity of the turkey with stuffing, such as breadcrumbs, sausage meat, sage and onion, etc.
Dressing is where you have the whole bird, and you have to remove the head, neck and feet, pluck the bird, and draw out the cavity, removing the giblets, and organs before washing it out with cold water, drying inside and out with kitchen paper.
Then the proper dressing is when you tie up the bird. Fully plucked, the ends of the leg joints are pushed into the cavity, the legs folded beside the brest and likewise with the wings. The string holds all the parts in place for stuffing or roasting.
Another term for dressing is removing all the bones of the bird to produce what is known as a turkey crown, but has no wings or legs attached and can be sliced when roated without any bones getting in the way.