Parallax scrolling allows background images to appear when the main body of the content scrolls up and down. it gives the effect of backdrop scenes being displayed.
Probably my favourite example of this still is the haunting site about the Aberfan disaster (opens in a new window)
It's a great way of story telling on a web page.
Some sites scroll the background at a slower speed to the top content, but that doesn't work on all mobile devices.