Parallax scrolling is a way to create a depth feeling using only 2D planes. Consider cutting out some shapes in cardboard. In the back you put the background, then you might add some trees in the next layer, then some grass etc. When you move sideways, you move the front layers faster and the back ones slowest relative to their distance, thus creating a sense of depth, South Park style. This is used in most platform games.
Edit: I was slow
