If you put something in the background, it will be projected onto a sphere that surrounds the camera. For example, if you put a bi-gradient into the background, it will show up in any direction that you point the camera. I commonly put a bi-gradient into the backdrop as it is also used for gi calculations. The backdrop is different. If you put something into the backdrop, whatever you put in there will be stretched into the backdrop of your render regardless of camera position, angle, or image render size. So, if you have an image that you want to exclusively be in the back of your image, then put it in the backdrop. I commonly put a white gradient into the background for GI lighting and a starfield map into the backdrop for my outer space renders. That allows me to get more filled and realistic lighting while "being" in the blackness of space.

-Kix