"Masking is one of the most important techniques or skills to learn in Photoshop. In this tutorial, we will explain 10 things you need to know about masking in Photoshop to help you apply interesting and exciting effects to your images. Let’s get started!"
http://psd.tutsplus.com/tutorials/tools-tips/10-things-you-need-to-know-about-masking-in-photoshop/
