Time Machine Question

From what i know Time Machine backs up everything you want it to everytime corect?
I am trying to find out if there is a way to make it so time machine is smart. Thus only backing up the changes that are made. So then i will not be making a completly new back up everytime i try to do it, and using up a tron of storage space.

Am i correct in saying that???? I have a friend who says her's only backs up the changes that are made each week or each time she backs up. Is that a setting i am missing.

Any help would be amazing.