The excellent Illustrator blog BittBox has a nice Adobe Illustrator Layer Mask tip/walkthough on how to use this highly useful, productivity-enhancing (and fairly obscure) feature of Illustrator CS and higher.

Layer Masks are basically clipping masks that apply to the entire layer (Layer masks need to be sub-layers, and the top-most one at that). The best feature is that they can be locked, and they are not tied to one specific object, or cause an entire group of unrelated objects to become “grouped” as they are when applying a clipping mask to them. This allows you to work normally with all the other objects on other sub-layers while still getting that clipping mask effect.

Head on over to BittBox and read the full post: Improve Your Illustrator Workflow with Layer Masking