Not to be confused with a quickmask, a layer mask is an addition to a layer that defines how transparent each part of the image should be. Of course, you can have parts of a layer be transparent without using a mask, but often times this is a lot easier.

Layer masks use white, black, and everything in between to define a value of opacity for every pixel of a layer. Black is fully transparent, and white is fully opaque.

After creating a layer mask, it will appear next to the original layer in the layers box. You can click its icon to edit it, or click the icon of the actual layer to edit that. Here, for example, I drew on the layer mask with a 19-pixel circular fuzzy brush, and you can see the effects on the image.

There are a number of options associated with layer masks. Apply Layer Mask takes the mask an adds it directly into the layer data, so that there are transparent pixels in the layer instead of opaque pixels in the layer that are marked transparent in the mask. Show Layer Mask displays the mask in place of the layer. The mask icon will have a green border. Toggling Edit Layer Mask is the same as selecting or deselecting the mask in the layers box. Disable Layer Mask is the same as clearing the mask to all white (opaque), or in other words not applying the mask. The mask icon will have a red border.