Quick masks are helpful for creating one-time selections. In addition, using a quick mask is an easy way to edit a selection using the painting tools.
As with any other selection, a quick mask selection disappears when you deselect it.
Alpha channels are stored in the Channels panel, along with the visible color channels.
You can paint in an alpha channel using black, white, and shades of gray.
Alpha channels are storage areas for saved selections. All layers set to be visible will be present in printed or exported output. But only the color channels are visible in printed or exported output; alpha channels are not. Layers contain information about image content, while alpha channels contain information about selections and masks.