In the field of Animation there are many different areas. You can be working with many different types of media and sources, but the principles that define animation are always the same. No matter if you are producing animation in 3D, 2D, Web, Clay or other.

First we have the 12 Principles of Animation, they are in no particular order.

Squash and Stretch, Staging, Straight-ahead vs Pose-to-Pose, Follow Through, Anticipation, Slow-in / Slow-out, Arcs, Secondary Action, Timing, Exaggeration, Solid drawing and Appeal.

Each of these have a definition of how they effect and affect a character or object that is being animated.

Each of these principles are fairly straight forward and donít require an extended explanation.

Squash and Stretch: Distortion of the form due to the transfer of energy.
Staging: Placement of the focus of your animation in logical special relations.
Straight Ahead vs Pose-to-pose: Two different ways of animating. (bonus: the best animators donít do one or the other.)
Follow Through: Object in motion tends to stay in motion.
Anticipation: Movements donít automatically happen, objects at rest tend to stay at rest, until energy is transferred.
Slow in / Slow out: Motions need to build up their acceleration due to forces that act upon objects, a car cannot jump from 0 to 90 instantly.
Arcs: Nothing moves in a perfectly straight line.
Secondary Action: Reaction of the initial movement on other connected objects.
Timing: Everything moves a certain rate and or speed, to mimic this requires timing.
Exaggeration: Push the movement further to create a more dynamic pose and or act. (bonus: very few animators can control this, lack of skill with this principle is the cause for much of the overacting in animation.)
Solid Drawing: Everything needs to be readable from the audiences point of view, just because it works in your mind does not make it a solid drawing.
Appeal: Everything must push the audience towards wanting to continue watching.

I was taught those 12 principles over and over throughout my schooling as the set of rules. But there is another set that I find to be equal if not better to these 12.

The 28 Principles of Animation.

Pose and Mood, Shape and Form, Anatomy, Model or Character, Weight, Line and Silhouette, Action and Reaction, Perspective, Direction, Tension, Planes, Solidity, Arcs, Squash and Stretch, Beat and Rhythm, Depth and Volume, Overlap and Follow-through, Timing, Working from extreme to extreme, Straights and Curves, Primary and secondary action, Staging and composition, Anticipation, Caricature, Details, Texture, Simplification, Positive and negative shapes.

These are even more straightforward and really require no definitions. The 28 take a more systematic dissection of what is in good animation, from a real standpoint. Anatomy dictates many of the other 28 principles. Weight influences the others as well.

So here is a conclusion to the principles of animation. As an animator I feel if you take these principles and specifically try to incorporate them into your animations every step of the way, you will not be a good animator ever. Your work will look over worked, contrived and relatively lack-luster, it will be predictable. These principles are not strict rules that you follow, but should be learned so your subconscious mind can use them as a set of markers. If as an animator you donít use most of these principles from common sense then the way to improve that part of yourself is to go outside and look at the world around you they are all there all the time in everything.