Turn A Layer Object Into A Mask (photoshop 6/7/cs)

Did you know you can turn a layer object into a mask? Using
another object as a mask is referred to as a clipping group. When
you group two or more layers, the base (bottommost) layer becomes
a mask for the entire group. Any texture or color layers above
the base layer are clipped so they only show on the base graphic.
This nifty feature allows you to control what your various layers
affect in an image. To group layers, you can link them and then
choose Layer > Group Linked. As an alternative, you can also
[option]-click ([Alt]-click in Windows) the line between the two layers.