I really don't know much about PS.

I think masks are a sub function which allows you to add another aspect to the function that you are already using. For example in the above tutorial, crypel drags in the sky then he adds a mask to the image, then he uses background/foregroud (or vice versa) gradient. I do use that technique all the time but that's the best I can exmplain it.

No idea on how to airbrush, what I do is use the blur tool on skin with airbrush selected. That can work very well.