The camera normally determines the optimal exposure given the semi-automatic or automatic settings selected by the user. There are times, however, where it is advantageous to capture images that are slightly under and over exposed. This is called bracketing. The primary uses of bracketing include a fairly new technique called HDR. Depending on the camera, the user may select from 3 to 9 images. One image will have the optimal exposure while the rest will be underexposed and over exposed by an amount determined by the user (usually 1 stop). Each camera has a special setting normally located in the settings are of the menu where the user selects various elements to set.
Also see: HDR.