Layer masks

[Project Shadow]

I keep hearing about layer masks, what are they?

[Gunslinger]

In a nutshell, a layer mask is simply a way to hide parts of a picture or text. Even though the masked parts are hidden they are still available if you need them. When using a layer mask always remember that black takes away and white adds it back.

http://en.wikipedia.org/wiki/Layers_%28digital_image_editing%29#Layer_Mask

[Project Shadow]

Oh, I thought a layer mask is when you make animation and you have to edit and layer underneath, you make those transparent.