Scott Steeves , Sep 12, 2003; 01:23 p.m.
From the Help:
Storing masks in alpha channels
In addition to the temporary masks of Quick Mask mode, you can create more permanent masks by storing them in alpha channels. This allows you to use the masks again in the same image or in a different image.
You can create an alpha channel in Photoshop and then add a mask to it. You can also save an existing selection in a Photoshop or ImageReady image as an alpha channel that will appear in the Channels palette in Photoshop.