In Photoshop what are layer masks?

Can someone please explain to me what are layer masks in Photoshop and what can they be used for?In Photoshop what are layer masks?
I'm not entirely sure what the other answers here are getting at, but I don't think they're describing the same concept I've seen in Photoshop. :S





A layer mask is kind of like an extra ';layer'; to your layer (for lack of a better word) which contains information about what parts of your layer should and shouldn't be visible. Layer masks can be either raster-based (i.e. made of pixels) or vector-based. For raster layer masks, a white pixel in the layer mask will make that layer's corresponding pixel completely opaque, while a black pixel in the mask will make it completely transparent. Gray tones will give you semi-transparent areas. With vector masks, you have a shape (or shapes) defined in the same way as other kinds of paths, and only areas of that layer which fall inside the shapes are made visible.





Layer masks are useful because they let you change a layer's colour content and transparency/opacity content completely independently of each other, providing a level of freedom you wouldn't have with a layer that simply has transparent pixels in its RGB channel. You can scribble all over your layer mask or move it around, and the actual colour and arrangement of those pixels will remain intact underneath. Conversely, you can mask off portions of a layer you want to be invisible, and you can scribble indiscriminately all over the layer itself without it ';colouring outside the lines';, so to speak.





You can add a mask to your layer by going into the Layer menu; there are several mask-related options therein. Hope that helps!In Photoshop what are layer masks?
The layers actually add/remove layers of colors to the image that is being used. You can see this demonstrated when you go to the layer section and scroll the bar from left to right. You will see that the the color will appear, then remove itself.
Here you have a simple example and which explains your question on layer masks





http://photoshoptips.net/2006/07/25/laye鈥?/a>








hope this helps !