What is the difference between a layer mask and clipping mask?

[GKLee]

My subject line pretty much sums up what I want to ask.

The thing is I've learned how to do both of them, but every textbook exercise I've done regarding one I found out that I can pull off with the other too.

Are they simply two different mechanisms for doing the same thing?

Thanks by the way.

 

[GKLee]

Any thoughts anyone?

 

[Nomission]

in a layer mask, you decide by painting from white (opaque) to black(transparent), the alpha level (transparency) of a layer, for instance, if you paint in a layer mask with a gray brush, those painted areas will became transparent to the level of gray you used.
A clipping mask, as the name says, is also a way of masking, but all that matters are the pixels of the masking layer, if there are no pixels in the cipping mask, the masked layer doesn't show. The main difference is that with a layer mask you mask what you want and it is linked with layer, the clipping mask is usually used when we have one pic to act as the mask of another and they are not linked (we can linked them, though)
Hope it helped...