Parallax scrolling

Parallax scrolling is technique often used to display backgrounds to create an illusion of depth : An image displaying a background element which is near the camera will scroll faster than a a background element which is far from the camera.

Here is an example : http://www.forum.compilgames.net/download/file.php?id=2941

With GDevelop, you just have to create some layers used for the backgrounds, and put the different backgrounds on it.
Then, instead of centering the camera of these layers on the object to follow ( let's call it MyObjectToFollow ), just put the X position of the camera of these layers to MyObjectToFollow.X()*0.25 for example ( and the same thing for Y if needed ).
The 0.25 factor used here tells that the background will follow MyObjectToFollow 4 times slower than usual ( 1/0.25 = 4 ). You can use others factors for the others backgrounds ( See the examples ).