Back full and D-Spin are essentially the same tricks. D-Spin is ever so slightly off axis and is usually done with the knees bent and making an X. A back full is pinned out and completely on axis.
Back double full is a backflip 720 completely on axis and pinned out. They usually tilt a little off axis on these just due to the 720 but for the most part they are on axis.
The big difference really is that you stay perfectly in line and layed out in full/double full.