http://www.techtv.com/screensavers/answerstips/story/0,24330,3399463,00.html
this should explain it =)
** Excerpts from article**
Alpha channels are masks through which you can apply effects. The alpha channel is an 8-bit channel, which means it has 256 levels of gray from 0 (black) to 255 (white). White acts as the selected area; black acts as the protected area. The level of gray in between determines the level of exposure. For example, 50 percent gray allows for 50 percent exposure. Alpha channels are basically specialized selection processes.
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“The beauty of these alpha channels is that they can be saved, modified, or combined to create special effects. When you use the Lasso, you make a selection. When you deselect, the selection is gone. If you Save Selection (Select > Save Selection), it is sent to an alpha channel that can be recalled as many times as you need.”
