You should really read my Animation Primer. The first post answers your question:
[Animation Primer (Part 1): Core Principles] - The 12 principles of animation and how we add the feeling of weight and speed to things that inherently have none.
[Animation Primer (Part 2): The Third Dimension] - The major benefits of moving from 2D to 3D and some basics on how we do it.
[Animation Primer (Part 3): It’s Automagic!] - Some ways we’ve figured out how to use technology and simulation to augment and improve our ability to animate movement.
Got a burning question you want answered?