Layer masking is one of the most powerful features that you could ever learn.  It is my single favorite Photoshop tool and I use it all of the time.  True professionals use the layer mask all of the time.  The layer mask is used to mask or "hide" pixels without having to erase them.  When you erase pixels, you are getting rid of them permanently (unless you have an original layer copy around) but with a layer mask you can not only "erase" pixels but you can bring them back and even fade them: you never "lose" anything with a layer mask!

Why would you want to use a layer mask?  Simply to hide pixels or make them invisible so you can see layers beneath, create a perfect blend, "erase" something that shouldn't be showing, etc.

My secret weapon?  Should I be telling this?  It's the gradient tool with the layer mask.  It will literally solve all of your problems!  It is how to create perfect blends and fades.  Since movie poster's are just about my favorite genre of design, I've found that 90% use (gradient) layer masking.  If you're someone who respects movie posters, then you should respect the power of the layer mask because now you have the power to create the same results as the best of the best!

If you understand how to use the gradient tool, you can use it on a layer mask to create perfect fades and blends.  You can also use brushes, paintbrush, or create selections and "fill".  It is important to note that when you create a layer mask you are creating a new channel of information that can be found under the, yep; channels palette.  When you are 'on' the layer mask you can also go to the channels palette and take a look at it. 

When you paint, fill or use the gradient tool with black as the foreground color you are going to be HIDING pixels on that layer.  When you paint or fill with white as the foreground color you are going to REVEAL pixels (if they were already hidden).  It may take a while for this concept to sink in but it is absolutely worth understanding.  When you work on a layer mask, (it has it's own channel) you use Black to White and any shades of Grey in between to reveal or hide pixels.  50% grey is exactly in between.  The closer shade of grey to white that you get, the more of the original pixels you will reveal (or "buy back" as I like to say).  The closer the shade of grey to black that you use, the more you will hide pixels.

You can easily create a layer mask by clicking on the layer mask icon near the bottom left of the layers palette.  You must switch back and forth between the layer itself and the layer mask.  You will goof this up many times because even I will "accidentally" find myself using a gradient on the layer itself; make sure you are on the layer mask when you want to be.  You will notice the link between the layer and the layer mask; this simply means that the layer and mask are linked together (almost always where you'd want it).

So get some practice fooling around with the layer mask, trying out the different tools (brush, paint bucket, fill and gradient tool) to hide and reveal pixels with Black, White and the different shades of grey.  You can toggle between white and black as your foreground colors by pressing "D" and "X".  What's the secret weapon again that pro designers use to get that perfect blend or fade of a layer?  That's right.  The gradient tool.  I will only use the brush (almost never paintbucket or "fill") when I need to get in tight and close on a layer to hide some edges for example; otherwise, I'll always use the gradient tool and switching between the different modes (using linear and radial the most). 

Make sure you understand the concept and application of layer masking and get some good experience because it is one of the most practical AND professional things you could ever learn in Photoshop and you will continuously use it as a graphic designer.  The Basic Photoshop training covers layer masking in application extensively.  You can also enable/disenable a layer mask, apply it (essentially erasing those areas "masked").

If an area of the layer is too invisible and you want to bring back the original pixels you a brush or radial gradient tool (which I use) with white as the foreground color to carefully bring back the pixels that you want visible.  Here is another practical example of layer masking where I am simply 'masking' or hiding pixels. 

Remember that the original layer is still there (as you can see if you disenable, fill with white or throw away the layer mask).  If you're not layer masking yet, YOU JUST DON'T KNOW. ;)

- Orion Williams copyright 2004