This isn't so much a straight "tutorial" as it is a small
lesson on masks. You'll learn to think for yourself with masks, as I'll describe what layer masks do, why you want to use them, and "how to".
WTF is a Mask?
We initially find a picture that has a subject and a background. We want to have the subject all by itself with no background, so we use a mask.
For example, I want to extract
Shannon Elizabeth out of this picture by "masking" or "covering up" the background so I only see her.
First thing to do is unlock and rename the background.
Double-click on the layer's name (background) inside the
layers palette and rename it "
original"
Now click
Layer - Layer Mask - Hide All. 
This puts a "mask" over the entire layer so you can't see anything! Check out your
layer palette again and see the little
"link" icon and
the big black square next to it.
That black signifies that the layer has been
completely masked, or hidden.
Select your
brush tool (
hotkey B), using these settings:
Make sure to set your colors (foreground, background) back to Default (white foreground, black background) by hitting
hotkey D.
Now, making sure you've first clicked on the little black
layer mask in the
layers palette, click and drag a brush stroke over the picture. Paint it anywhere and watch what happens!
Now, take a look at the
layer mask again in the
layers palette.
You can see a
white area that corresponds to what is exposed in your picture now! On the
layer mask (not the picture itself), when you
paint with white, it
exposes the layer underneath, and when you
paint with black, it
hides it again.
This can be used in a variety of different ways, but I wanted to simply expose you to the idea of masking. In the next steps we'll start identifying different methods of masking and the reasons for using each.
With a little playing, here's the finished, masked product.
Various ways to use Layer Masks 
Crazy Closet
Funky Pong
Snake
Chopper Challenge
Curveball
Linear Mode
