Whether you know if or not, you have likely encountered parallax scrolling. ESPN has used it in some of their longer articles; The New York Times, The Verve, and Pitchfork have also utilized it.
If you're not familiar with the term, parallax is a design element that uses multiple layers that create an illusion of depth and movement, kind of like turning a page. But, like most things, parallax has both good and a bad sides.
What's good about parallex is that it can create a really cool design effect. Normally, we would paste images of examples that we refer to. But these are all about the scrolling and experience, I'm going to list a few examples that utilize parallex so that you can see them for yourself: