The Mighty Alpha Channel
Photoshop has one other type of channel: alpha channels. Their special purpose is to store selections so you can use or edit them later.
These channels get their name from a process called alpha compositing, which combines a partially transparent image with another image. (Filmmakers use this process to create special effects and fake backdrops.) Information about the shape of the transparent area and the pixels’ level of transparency has to be stored somewhere, and that somewhere is an alpha channel.
This is powerful stuff because the same technology lets you save selections. And, as you’ve learned, making selections can take a ton of time. (Heck, you may not have the stamina to finish creating a particularly challenging selection in one sitting!) And since clients change their minds occasionally—”Put the model in front of this bush, and change her hair color while you’re at it”—the ability to save selections so you can mess with them later is a lifesaver. Likewise, a saved selection is crucial for making the Content-Aware Scale feature behave because the selection lets you tell Photoshop not to mess with certain areas in your image (see Content-Aware Scaling for the full scoop). As long as you save your document as a Photoshop file (File Formats), that alpha channel will always be there for you to use. That ought to make you sleep better at night!
You can drag alpha channels between documents as long as both documents have the exact same pixel dimensions. ...