Masks are an incredibly versatile design tool that is somewhat complicated to understand. Until now. The term comes masking is a lot more familiar when it is in reference to painting. Yep, I am talking about masking tape. You tape molding and windows to protect certain parts of a wall from being painted. We are using masks the exact same way by hiding parts of an image, graphic or effect for design ascetics. Here are couple videos to show how masks can be used in Darkroom and a bonus video for you Photoshop users.
Here are a couple sample files to help you get started. Download Sample FilesPart 1