Parallax scrolling

You've already seen the effect and maybe didn't know that it is called parallax scrolling--it's a popular visual effect where your background image moves slower than the image inn the foreground, allowing you to dramatically reveal content, as in this example:

It's not hard to do if you are using a platform like Wix, which provides this tutorial, Captivate Your Site Viewers with Parallax Scrolling.

If you are using Wordpress you'll have the option of using either themes or plug-ins to create this effect. Here's a handy guide.