Is a javascript framework allowing you to slide in pages of your site with having a parallaxing background. Parallax.js keeps pages in a structureless format, meaning you can absolutely control where pages come in and from where, even if it doesn't make geometric sense. You add html elements, or pages, to Parallax.js and use the movement functions to slide them into view, pushing whatever was there out of the way.
Check out the demo page's features section. It should explain everything you need to know to use Parallax.js. I strongly you suggest to take a dive into the demo page's code base as well, some nifty things in there.