Parallax scrolling is an effect in websites where background moves slower/faster than foreground. Parallax tricks us, gives an illusion of depth in a webpage. It is a nice effect. Adds a feel good factor to the website.
Parallax scrolling effect can be pushed further, you can have multiple layers in the page section and each layer moving at different speed.
The layer that moves slowest will give you an illusion that it is farthest, and the one that is moving faster is closer to your eye.
My first exposure to parallax scrolling was a Nike site, they had foot-wears in the background moving at different speed. Very cleverly blurred the ones that was moving slow; giving an illusion that it is too far away from your eye sight.
It is so interesting to see how our web technology is evolving and how creative creators get even with the simplest of effects.