Question: What is a mask? How do we use it? What exactly does it do?

A mask is a wonderful tool! While I have used it consistantly for over a year, I have only just begun to understand all the possibilities it has. A mask allows you to put one layer on another, yet see the bottom layer through the top one. It is similar to the erase tool, but with more ability. In your layer pallette, highlight the image you want to maks, then click on the mask box at the bottom<gray box, white circle>, which will give you a white box next to your thumbnailed image. That white box is the mask. Select your paintbrush and a size<the bigger the brush, the more it will erase, I usually start with 100>, and make sure that your forground color is black. 

Now click or drag around the part you want to erase, it will completely disappear. You can also set the opacity of the brush, the lighter<50% etc> the less heavy it will erase, leaving some of the image visible. You can use brushes downloaded from the web as well as the basic ones. If you find you have erased too much, make the foreground color white and then you can "erase" your image back in. Also, you are able to change the blending mode of the mask via the layer pallette> Once you learn masking, you will not go back :) By Val.