Lots of people think using a layer mask is only for the advanced Gimp users, fortunately this isn't the case at all. Let me explain you the basic usage of layer mask on a easy to follow step-by-step example.
Pretty nice looking and simple to create effect, where we turn everything into pixels except the selected part. My car will end up looking like it's trying to escape out of the pixelated background.
If you found yourself in a position where you wanted to crop a certain photo into a perfect square but you just couldn't figure it out how, than this tutorial is for you.