This is another cool tutorial that I came across. It really helped me better understand layer masking and how to lighten and darken only certain parts of my pictures. As you can see, its purpose is to help you take an ordinary picture (see below)
and turn it into something spectacular like this (see below)
Check out this very impressive tutorial at: http://www.photoshopsupport.com/tutorials/masking-and-montage/photoshop-masks.html