Layer masks
From The Glossary of Digital Photography
A technique used in Photoshop to control how a layer is applied to an image. On a layer mask, painting in black will block that area of a layer from being applied to the background image. Painting in white will allow that part of the layer to be applied. Varying shades of gray will allow some of the layer to be applied. A photographer could create a layer that was very blurred, then use a layer mask to block parts of it so that the underlying portrait would have, for example, sharp eyes and lips. The layer mask is a very powerful tool in Photoshop because it does not change the underlying image, only the layer. When the image is saved with layers intact (not flattened), the image can be reopened and reedited at any time without losing information. See also layers
