How to Use Clipping Mask?
Clipping mask is another way to create transparent masks. It is uniting several layers into so called clipping mask.
Creating Clipping mask:
> Create a new document (File -> New)
with transparent background.
> Create another New layer.
> Insert an image.
> Switch to the bottom transparent layer and paint anything.
> By pressing ALT
key position the pointer over the line dividing two layers in the layers palette.