If you're a "Photoshop Guru" (as my computer teacher called them) then you are laughing at this question. My response to you: sit down and shut up so I can explain it to the people who AREN'T Photoshop gurus.
PLEASE NOTE! Not ALL advanced image editing software allow for use of masks, and some implement the use different that described in the article. Don't say I didn't warn you.
A real mask allows you to cover your face and conceal it from people. Further more, you might only where a masks that reveals part of your face.
Let's say you're going to a formal dance where people are dressing in costumes. And low and behold, you have... A PIMPLE! It's smack dab in the middle of your forehead. A mask would be keen, but you don't want to completely hide your face for that special someone you are looking forward to dancing with. So you put on a mask that covers the top of your face, leaving the bottom exposed for spreading gossip, whispering sweet nothings in the significant other's ears, all that mish-mosh. Basically, it exposes the part of your face you WANT people to see, and hides the part you DON'T want them to see.
That's what a mask does for images. It can be used to show parts of a layer you want to show, and cut out parts you don't want to show. Sounds suspiciously a lot like just using the eraser tool... BUT IT'S NOT!
A mask (well, the one in GIMP anyway) uses two colors. One is the transparent color: that is, the color you use to hide parts of the layer. The other is the OPAQUE color: the color you use to show parts of a layer. You block out or expose these parts by applying these colors to the mask.
Lets say I want to put two photo's together. The first photo I'll use is one of this eagle I got off of Google:
And let's say I want to place it in these mountains (note that they've been scaled down to fit on this page. The real image is much larger):
Now, I put the two images together. I simply place the eagle on top of the mountain in a new layer.
Looks VERY crappy. It's obvious that the two were put together. Look at the box surrounding the image. Blah. So, I'm going to make it a little more convincing (not totally convincing, else I would have to do color correction, lightsource correction, the whole nine yards). To do this, I apply a mask to the layer that the eagle is on. I set it to be completely opaque (which means for now, the layer still looks the same). Then, I grab my designated transparency color (in this case, black) and apply to the blue area surrounding the eagle. After some careful painting, I have the following:
You're probably thinking "big woop! You can do the same with the eraser!"
Well, yes... you can. But lets say... there was a mistake...
OH NO! My little brother got to the computer and (assuming he didn't change the tool) started to mess with my image! And now, it looks like this:
Now, if you were using the eraser tool like I told you NOT to, you might be frantically pressing the "undo" button. And if you have your program set to a limited undo history and it didn't undo everything, then you would be screwed.
But since we're on a mask, all you have to do is switch to the 'opaque' color and all is groovy.
Assuming you only use the 'opaque' and 'transparent' colors that your program designates for using on masks, you can reverse any painting you've done on a mask by switching the two colors. So, if I wanted to, I could switch colors, go back to the image without messing with the 'undo' history and do this:
That is very cool. Yes it is. Praises are do. Show me appreciation. Give me money
....Just kidding <:3
Anyway, this will come in handy later. We'll apply it to our colored images for interesting results. It may seem useless at first, but if you're willing to work with me and use the method I present, it'll make things a little easier on your sanity.