It is confusing Urban Girl but a good question that I pondered on a few years ago too.

Technically the differences are that a designer has studied longer than a decorator and has a higher qualification. They complete units on architecture that a decorator doesn't.
In the work field technically a decorator only works with the interior of a building. The designer however can work with the structure of the building.
For many years I called myself a decorator out of respect for those designers who have done the extra study. But residential clients aren't interested in the differences. I also work with the structure of building by using my licensed building. We frequently move walls and doorways when renovating a kitchen.
So in the end I don't think it matters - as long as you have happy clients at the end.