Learning non-destructive editing techniques helps you produce documents that bend along with your creativity. Photoshop Masks are the cornerstone of this process. Not only do they preserve important pixel data, but they allow for the creation of flexible interface elements as well. In this article on Smashing Magazine, it talks about the technical aspects and creative advantages of incorporating masks into your workflow, it also define five methods of masking: Pixel Masks, Vector Masks, Quick Masks, Clipping Masks and Clipping Paths. It’s worth to spend the time to read this article!