For a long time cartoons relied on their characters to bring their world to life and to give the illusion of their world being 3 dimensional. The only problem was when the character left the shot it became very apparent how flat the scene was. So in 1937 Disney invented the Multiplane Camera.
This device allowed backgrounds to move in a parallax motion so to give the illusion that everything is moving relative to the viewer. This really allowed their cartoon worlds to appear 3 dimensional just with the background artwork alone.
It worked by breaking up the backgrounds and painting them on multiple panes of glass with oil paints. The individual planes could be manually moved where desired. The planes closer to the camera would move more often and more quickly than the ones further away to mimic a real landscape. Once all the frames were taken and played back they would show an animated background.
Nowadays this effect is easy to recreate digitally with video editing software. The background planes just have to be placed on overlapping layers. This is a parallax test I did with an early version of my background in After Effects.