Photoshop: Clipping Masks

So you have an idea of what a layer mask is and how it works (as illustrated below) and want to add another to your set of techniques.

The next one you might want to learn about is called a Clipping Mask.

A clipping mask allows you to show or hide selected parts of an image depending on the pixels above it.

Hold down Alt while click between the layers in the Layers palette. You need to be able to see the double circle cursor.

The Duck layer in this example has some transparent pixels. You can see this by the checkerboard design in the thumbnail behind the yellow duck.

The non-transparent pixels reveal the pixels of the layer above it. So the Kaleidoscope layer is clipped where there are no pixels in the layer below.

It might help to think of the pixels above being caught by the layer pixels below and everything else just falls away.

Here is the Adobe Photoshop manual explanation:
Clipping Masks