Cut outs are a class of stock photography, illustrations, or 3D renders that have an object placed against a solid-colored background, usually white, black or color-keyed green.
On 123RF.com, a stock photography store, there is a filter for cut outs so you can get images that are set to allow removal of the background color and to overlay it on another image. When creating book covers, for example, you don't want your book cover to look like anyone else's. By taking a cut out 3D rendered space ship and overlaying it on a NASA Hubble astrophotography image of Jupiter, you can create a unique cover that makes your science fiction novel stand out. As an example, I used this method to create a cover for Harl Vincent's book, The Adventures of the Vagabonds, available at a bookstore near you.
When using cut outs, you can use image manipulation software such as Photoshop to select the background and delete it, creating a transparent area around the cut out object. By overlaying it on another layer, the cut out object appears to be in the background environment.