A 2D animation or 2-Dimensional animation is basically an animation that took place in a two-dimensional space. Modern 2D animation can be created by hand or computer animation software. The movement of the object requires one image to be followed by another with slight different position, followed by another image in another position, and so on. It is a traditional animation drawn with simple motions. The objects can only move in two axes, either up or down, left or right.
They are generated by creating continuous consecutive images, or “frames”, thus forming motion by each image showing the next in a gradual progression of steps. Unlike 3D animations, 2D animations are flat images that can move, and represented by height and width dimensions but not by depth.
3D animation, or 3 Dimensional animation is an animation drawn in three-dimensional space. The objects can maneuver in three axes rather than two as in 2D animations. 3-D animations like any other 3-D movies you may have seen, allows you to create a realistic objects that are more exciting. Different textures and lighting are applied to the objects created that appear solid, and can be applied seamlessly into live video elements. However, to create a beautiful, realistic 3-D animation, can only be limited by the capabilities within the program itself. A lot of time, effort and resources are greatly needed towards building a successful 3-D animation, and the returns may not be necessarily as expected.
2.5D animation is a 2D animation drawn into a 3-D space as it involves the motion of 2D-animated object in a 3-D space. The trick lies behind the perspective and shadows of 2D-animated objects drawn in 2D-space that appears to be 3-D. Animators use the impression of drawing 2D objects in motion that appears to be moving in a 3D space accomplished by using brilliant art skills in layering, shadows, perspective animations, morphing, and several other techniques. The result would create a fluidity movement changes in depth that deceived us into looking a 3-D animations when it was actually using 2D-animated objects. 2.5D effect can be rendered by using shadows applied to a 2D objects.
This can be achieved by having the drawn object casting a shadow behind it on the background. Simple example would be drawing any object on a white, plain piece of paper, and then add in shadows that stretches behind it into the distance, a dimension that does not actually exist for this object unlike 3-D which have 3 axes, but we can imply.
Your perspective on the object from 2D now appears to look like 3-D simply by adding a shadow behind it. Programs such as Adobe After Effects allows you to generate 2.5D effects by creating the false perspective, and then animate that perspective changing for 2.5D effect.
In short the difference between 2D, 2.5D and 3D is about to their depth that our eyes detect. 2D animation feels flat, 2.5D animation feels like it has some kind of depth and 3D animation give us a feeling that there is 100% depth from every angle that camera follows.