Hills is a screensaver which shows an animation with hills which seem to be covered with grass, but it is very far from providing a realistic effect. In fact, the hills look like those of an alien planet with a strange soil. The screensaver can be run in full screen or you can view it in a windowed pane. The animation can be stopped at any time and instead of seeing the green ground, you can also view a wire frame, which for me isn't attractive either.
From the Preference window, you will be able to adjust the full screen resolution and customize the animation properties, such as hills' height, speed, look ahead distance, camera height, grid size, fog density and fog color. Sadly, the screensaver also lacks sounds and music, which are the only things that could make the screensaver a bit more attractive.
All in all, I really don't know who may want to decorate their screen with Hills, but, in my opinion, it isn't worth trying, since you can get lots of beautiful screensavers for free.