WOW, that's deep man! parallax.js reacts to the orientation of your smart device, offsetting layers depending on their depth within a scene... Oh, you don't have a smart device? No worries, if no gyroscope or motion detection hardware is available, parallax.js uses the position of your cursor instead. Radical.
For the best experience, check out this site on a mobile or tablet equipped with a gyroscope
(iPads are the coolest)