Kyle's Layer Masks

In one of Kyle's classes they learned about what you can do with layer masks in Photoshop. He had an assignment to make a composition out of 5-6 photos using layer masks. Here's what he came up with:

I think it's really cool. Kyle showed me how to do this, but I've already forgotten. I need him to give me a refresher and maybe I'll try one for myself. I really like this, but Kyle feels like it's missing something, so he says he might change it, but this is what it is for now.