I use photoshop elements, so there is no masking tool. But I do use masking brushes. The idea is to create an area that a layer wll show up in, without showing anywhere else. That was a really bad explaination.
Anyways, I don't know all the benifits of masking, as I've ever used the actually tool.
However, I use the brushes by having the layer I want to 'mask' set to screen. Then create a layer underneath that, and fill it with white. Then get your brush and, in black, brush where you want the image to show. It's fairly simple.
A good tut on proper masking can be found
here.
I hope that helped somewhat.
!