Do not be fooled by the seemingly simplicity of this question.
I was actually quite tempted not to write this article because it's such a difficult topic to write on. It can be answered as complicated as you want it to be or as short as you desire. The thing is, I need to let clients know how complicated the process is, but not to let them walk away confused. Saying that, I will try my best to answer this question in the most concise but uncomplicated way possible.
In a nutshell, computer 3D animation refers to the work of creating moving pictures in a digital environment that is three-dimensional. Through the careful manipulation of objects (3D models) within the 3D software, we can then export picture sequences which will give the illusion of movement (animation) based on how we manipulate the objects.
What happens in animation is that motion is simulated in a way that the eyes tend to believe that actual motion has taken place while the fact is the perceived sense of motion is only because of the consecutive images that are passed through very fast.
This theory is inherent be it for 3D, 2D or stop motion animation.
In traditional 2D animation, pictures are hand-drawn and every one showing subtle changes from the previous. When played back sequentially, it creates the illusion of motion. In stop motion animation, real life models are moved slightly and filmed. Again, the pictures will create the illusion of motion when played back. In 3D animation, everything is done within the computer and exported from the computer.
The process of creating 3D animation can be sequentially divided into three phases: modeling – which describes the process of creating the 3D objects within a scene, layout and animation – which describes how objects are positioned and animated within a scene, and rendering – which describes the final output of the completed computer graphics. Through the combination of the above phases and a few other sub-phases, this completes the process of a 3D animation production.
There are many software in the market for creating 3D animation, ranging from the cheaper lower-end ones to the professional high-end versions. If you are curious to see how a 3D software works, you can download a free one called Blender. Just Google it and you will be able to find its official website.