(via Terry White )
Layer masks are something experienced photographers and long time Photoshoppers use and love. They can help in a lot of different ways and more importantly, masks do not damage or remove pixels in your image.
Even so, for less experienced Photoshop users, layer masks can be a little challenging, especially if you don't understand how they work.
The word "mask" can be confusing, that's why you can mentally replace it with "transparency” . That's basically what masks influence. Now this might raise questions as well because, as you know, you can already influence a layer's transparency using the opacity option. The major difference is that when you change transparency using the opacity setting, you do it for the entire layer. That works great in some situations, but what do you do when you only want a part of the layer to be more or less transparent? That's where masks come in handy because you can use them to alter the transparency of any section of the frame.
In a quick video tutorial, Terry White explains how layer masks work and he also shows it with two examples. If you've ever wondered how magazine logos are positioned in front of the cover photo, this tutorial might have the answer.
(Success Tip:Take better photos with this simple deck of cards)
Also, layer masks are one of the many ways you can adjust exposure in post processing, yet unlike other methods, using them isn't harmful to the file.