What’s the difference between Objects and Backgrounds?

Objects and Backgrounds are both Sub-Types in the Design Elements category. 

Objects: Elements with irregular borders and transparent areas. They can be vector files or raster/bitmap files with transparency (PNG or GIF). Examples include icons, logos, badges, shapes, or cut-outs of photographed elements.

Backgrounds: Full-frame imagery designed to serve as a canvas for text or to give context, texture, and depth to a layout. These may be project files, vector files, or transparency files. A non-transparent JPEG will remain in the Photo product type. Non-Transparent JPEGs may be included in a Design Element pack as an "extra" with the other primary file types."