This helped me a lot
So, as far as I understand, mask helps us to "paint" areas on the layer where we dont want to apply the effect? Or Im just confused :S
I would have a hard time explaining what a Layer Mask actually *is*, besides being a tool to do something with; much like I would have a hard time explaining what a hammer actually *is*, besides being a tool to do something with. The best way to understand either of these things, in my opinion, is to jump in and actually start
working with them. To that end, follow the steps in the first article I linked you to, "Understanding Layers in Photoshop". Still, I'll do my best to explain how Layer Masks work...
The Layer Mask itself is "transparent". You "Paint" on the Layer Mask using the Photoshop Brush Tool in either black or white. The Layer Mask is what allows you to selectively apply the effects of different Adjustment Layers while using the Brush. The Brush Tool has two functions: painting in Black which conceals (hides) the effect of the Adjustment Layer, and painting in White reveals (shows) the effect.
Layer masks are white by default, which means they automatically allow the adjustment layer to affect the entire image. With a White Layer Mask you would use a Black Brush to
conceal those parts of the Adjustment Layer you don't want seen in the final image. Or, you can switch to a Black Layer Mask and use a White Brush to
reveal those parts of the Adjustment Layer you want seen in the final image.