You can use the Auto-Align Layers command from the Edit menu to make a composite photo from a pair of nearly identical images that may contain some unwanted areas. For example, one shot of a group portrait is ideal except that one of the subjects has her eyes closed. In another shot her eyes are open. Using Auto-Align Layers and layer masking, you can combine these shots and eliminate the flaw in the final image.
For a video on aligning layers by content, see www.adobe.com/go/vid0014.