What is the Parallax scrolling effect ?

Parallax scrolling is a technique in computer graphics and web design, where background images move by the camera slower than foreground images, creating an illusion of depth in a 2D scene and adding to the immersion.

Web designers began incorporating parallax scrolling in 2011, using HTML5 and CSS3. Proponents use parallax backgrounds as a tool to better engage users and improve the overall experience that a website provides.