I have written an eBook that teaches a lot more about layer masks. Get it here: http://amzn.to/17bcMiS
Original Post: http://gimpedtutorials.com/index.php/layer-masks-explained-lesson-5-how-to-use-gimp-for-beginners/
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This lesson explains how layer masks work, and will give you an open introduction to the power this tool can bring to your photo-editing. This is probably the most important lesson on this site, because without this tool you will never be able to effectively get the more complicated edits down.