I know for myself Layers have been come a large part of what I work with in photoshop since the ability entered photoshop 20+ years ago and the use of being able to Mask objects has only enhanced things further – though using masks can be tricky at times (and I know for myself in the early years I never fully got how to do it, but as the year have gone by its gotten easier to work with). The following site is a good option for those who are looking to learn about Masking it might be semi out of date being published in 2011 but the basics are still there regardless.
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Layers are probably the single most important addition to Photoshop since the original version, but layer masks are a close second. I would posit that until you thoroughly understand how and why to use masks, you simply don’t understand the power of Photoshop.
A Complete Beginner’s Guide to Masking in Photoshop – http://designshack.net/articles/graphics/a-complete-beginners-guide-to-masking-in-photoshop/