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.
Watermarks are a great way to protect your photographs if you don't want other people misusing them. We will create a simple but effective text and turn into a Gimp brush so you will be able to stamp your work.