The great thing about Photoshop is that we can even combine a grayscale layer mask with a vector mask:

 

There is something about the above example that is important to know and that is that vector masks have a higher priority than layer masks. Look at this image:

 

With the regular layer mask active and the vector mask disabled you'll notice that the black line in the layer mask affects the complete layer. With the vector mask enabled and the layer mask disabled you'll see a green circle.

With both masks active you'll see in the previous image that the layer mask is restricted to the vector mask; the black line only affects the green circle.

You'll also notice that we now have a link symbol between the fill layer and the layer mask (you can move the mask independently of the layer by turning this off) and a link symbol between the layer mask and vector mask (you can move the vector mask independently of the layer by turning this off).

It's important to know that the masks are linked to the layer and not each other, meaning that both masks can be moved independently of each other, even if all link symbols are visible.

You can also add a mask to a layer set.
This makes it for example possible to combine layer masks as shown in the next screenshot.
This layer set mask will also effect all layers that are inside nested layers.

A clipping layer is nothing but a layer that is restricted by the layer beneath it. A layer can easily turned into a clipping layer by pressing Ctrl+G (command + G on the Mac) or Ctrl + Alt + G (command + option + G on the Mac) if you're using Photoshop CS2. The clipping layer will only affect all non-transparent pixels. In this example the gradient only affected the circle:

One of the best things of layer masks is that we can apply a effect to them. In this example the effect distort / glass was applied:

Masks can be used with adjustment layers. Have you ever added a new adjustment layer and did you notice a white square on the right of it in the layers palette; well, that's a mask. By default Photoshop adds a white mask (all visible) to every new adjustment layer.

Look at this example:

.

We have a background filled with a gradient and on top of that we have a box that has a layer mask attached to it. The layer mask has made the white background of the box layer invisible and that's why we're now looking at the background gradient of the background layer.

In the next example we've activated the Hue & Saturation adjustment layer that is clipped to the box layer(Ctrl+G ( Command + G on the Mac) or Ctrl + Alt + G ( Command + Option + G on the Mac) if you're using Photoshop CS2), so that it will only affect that particular layer:

Notice how we used the Hue & Saturation adjustment layer to change the color of the box to blue. Its mask was disabled.

In the next example we've activated the mask that contains a linear gradient going from white (top) to black (bottom). Remember what I said earlier; lighter grays decrease the transparency, darker grays increase the transparency. The mask has used this to affect the result of the original Hue & Saturation adjustment layer.

One way to create a mask is by using a selection. In this example I've selected a paper box:

 

When we now click on the New Mask icon , Photoshop will add a mask that is based on this selection:

 

Important to know is that you can also invert a mask. In the next image I've selected the mask (notice the double border) and pressed Ctrl + i (Command + i on the Mac):

You can also invert an image using the menu: Image / Adjustments / Invert.

If we don't change the mask after we've created it, then we can use this mask to return to our original selection. This is done by a Ctrl + left mouse click on the layer mask (Ctrl is command key on the Mac). The layer doesn't need to be active for this.
Being able to convert a mask to a selection is a great advantage, because it allows us to save a selection without the need to save it in a channel. It's also an advantage to see what is selected and masks allow us to do this.

If you have created a mask based on a selection and want to save this mask because you're planning to make adjustments to it, then either save it as a channel or duplicate the layer and make it invisible.

	PC	Mac
Select Composite	Ctrl + ~	Command + ~
Select Layer Mask	Ctrl + \	Command + \
Load Composite as selection	Ctrl + Alt + ~	Command + Option + ~
Load Mask as selection	Ctrl + Alt + \	Command + Option + \
Layer Mask as Rubylith	\

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