Michael Lackner shows you how render layers work in this 21 minute tutorial. Render layers are an immensely useful tool for rendering and compositing that allow you to separate elements in the render into separate layers you can work with individually in the node editor. Be sure to check out his youtube channel below as well, he's got a lot of other great videos and tutorials.
Two years ago, when I started with Blender, renderlayers were something mystical for me. Now I use them quite often and they are just like adding a cube. But I'm sure, that isn't the case for everyone, especially for the beginners, so I decided to make tutorial for you guys out there.
I hope you like it!