The Parallax Effect

3D animation takes place in a three-dimensional space with objects able to move on all three axes (x,y,z), as opposed to the the 2 axes (x,y) 2D allows.

But wait... What is 2.5D animation? 

Put simply, 2.5D animation is a 2D animation in a three-dimensional space.

The parallax effect allows motion in still photographs. One can accomplish the parallax effect by taking a still, separating it into different layers, then putting those photos in a 3D space and moving a camera through it. This also involves using perspective and shadows to make those 2D objects look seemingly three-dimensional.

The parallax technique brings still images to life. It's useful when trying to replicate live footage. The following is a great example and short tutorial of how it works: