Warning
This plugin is not maintained. Note that it has been possible to make parallax effects in near-pure CSS for some time, using a combination of translateZ(), perspective, and either a bit of JS for pointer tracking or a scrolling container. It is srongly suggested that you explore that avenue, as the resulting animations are optimised for the GPU, and smoother than this plugin can ever be. There is parallax scrolling page that I made based on this technique here
jquery.parallax turns nodes into absolutely positioned layers that move in response to the mouse. Depending on their dimensions, and on the options passed in, these layers move at different rates, in a parallaxy kind of way.