I believe what you are after is Parallax. What you do is, you put the background on a separate layer. And under that layers 'Layer properties' there's a thing called 'Parallax'. As default it's set at '100, 100'. If you want the background to scroll slower than the foreground; lower the values to e,g '50, 50'.
Give it a go and test for you self. It's really easy to set up.