Hi there! As an animator, I often asked to share my experience in animation. Since I'm also a lecturer that teach Lightwave and Character Animation, I would love to share it to everyone. Enjoy!
I would divide 12 principles to first 3 principles which I think is the most important things to understand for beginner :
1. Timing. This is the most important thing. You should be able to judge timing of the movement of everything you see in reality and duplicate it in your animation. With timing you can make your object look sad, heavy, light, happy etc with only a good timing. Nothing in this world would share same timing / pace and there should be a slight differences for every object. Say, A heavy object would take different timing than a light one.
Look at the examples:
Light ( 815 KB, mpeg1 )
Heavy ( 269 KB, mpeg1 )
2. Slow In and Slow out. Or Ease in Ease Out. It's the natural thing in this world where object would start moving and stop gradually not abrupt. Except in some extreme cases. Every object has a momentum and according to Newton law, every object would try to keep current state. Uh, now we're talking physics...
Look at the examples:
With Slow In / Out ( 399 KB, mpeg1 )
Without Slow In / Out ( 363 KB, mpeg1 )
3. Arcs. A movement would naturally go with arcs not straight.Look at your arm, notice how it move when you want to reach something..it would moving in arcs because they has joints. Try to throw rock ( be careful not to break your neighbor glass ) and notice how it fly with arcs.
With Arcs (271 KB, mpeg1 )
Without Arcs ( 295 KB, mpeg1 )
Now we go to other 5 principles
4. Follow Through and Overlapping. It's most obvious in a character_that_have_many_limbs. Dog for example. Try this: grab enormous dog food then go find a big and thick wood panel. Don't forget to have your favourite camcorder too. Call your dog, show him the dog food ( for best result, don't feed your dog at least a day ). Quickly push start button on your camcorder, and when your dog ( should be running like crazy after you, if not, you may be not feed it for a week, you must be totally a psycho ) at a point critical near you, put the big thick wood panel in front of it. Bam. Now rewind your tape ( make sure it's inserted before you roll the camcorder, if not, have your dog ram to you again ) then play it in 1/4 speed. See when your dog hit the wood panel, the head stop, the neck still move forward a bit then bend, the body also move forward then may bent upward, the tail sway upward following belly, and so on. The point is, nothing stops in one time, and they always do some overshoot because a momentum they carried. OK, I'm just kidding, don't do this to your dog........try your neighbor's dog...... ( just kiddin' again )
With Follow Thru and Overlapping ( 815 KB, mpeg1 )
Without Follow Thru and OverLapping ( 701 KB, mpeg1 )
5. Secondary Action. It's, cleverly, not a main movement. Ever saw people standing in a Bus Stop ? They never standing still, they do some small action. The context is standing waiting for bus, but they might scratch their head, or move their jacket form left to right, etc. Got it? Just a small action. Even a breathe is considered Secondary Action.
With Secondary Action ( 581 KB, mpeg1 )
Without Secondary Action ( 24 KB, jpeg, this one is what you see in a very stiff robot all the time, so let's save bandwith :)
6. Squash and strecth. Ha! This one is tough to see in realistic world. It may do some squash and stretch but in very small amount that your eyes will never noticed. My eyes also. Except very flabby thing.
With Squash and Strecth ( 815 KB, mpeg1 )
7. Exaggeration. The heart of cartoony animation. It take ordinary animation to a level where it catch the eye. I believe you already know this. If not, you wouldn't bother learning animation. Two sample below is just simple examples. One is ordinary move, it's appeared right because it has secondary movement, overlapping etc but lacks of eye catching-ish thing. Other example is exaggerated version.
Ordinary ( 685 KB, mpeg1 )
Exaggerated ( 1.323 KB, mpeg1 )
8. Straight Ahead and Pose to pose. This one is addressed for 2D animation. But we can translate it for 3D where we work in stages. I always work in stages. Means that I do some rough timing, then if it's considered okay, I refine and refine and refine, and save it in different name. This way, I can match the time needed first and refine whatever necessary. It is considered Pose to pose style. While straight ahead is a style when you just do animation in detail from at very beginning.
Stage01 ( 465 KB, mpeg1 )
Stage02 ( 1.317 KB, mpeg1 )
Stage03 ( 1.317 KB, mpeg1 )
9. Anticipation. A movement or act or scene preceeding another scene or act or movement. The purpose is to make viewer 'aware' of what will happen next ( and hopefully will hold a breath ). It's common in Horror movie that a character have back shoot camera following that character. Purpose? For creating tense. Or in Cartoon movie when a character jump off a cliff it usually do some hopeless grin, the "I'm doomed" style, then fall behind. Or in the example below, a character sway far back then.....Bam!
Anticipation ( 249 KB, mpeg1 )
Now we reach the last 3 principles
10. Staging. This principle isn't directly related with motion, as it more visually related. The most famous example would be 'magician pose', if you recall ( and if you ever watched one ), the magician always stand with most of his gesture visible. This meant for that viewer will easily understand what is happening ( and hide his/her tricks as well ).
Let's see sample no. 1
Now let's see sample no.2
11. Personality. A character should have distinct from the other character. I believe that no one in this planet in the same time share same personality. Even twins has different personality no matter small it is. If we clearly define what personality a character has, it will drive the character to move in its own movement characteristic. If we achieve this, we will have a character that looks like its move on it own thought, not just a puppet anymore. In samples you could see two different personalities on a same character.
Cocky ( 1.323 KB, mpeg1, same as exaggerated sample above )
Dumb (1.183 KB, mpeg1 )
12. Appeal. Well, this one is a bit difficult to explain, as it more 'feeling' related. But let's see previous example, where a robot walking in cocky style. It has more appeal than 'mere' walking, no matter good the cycle or secondary or follow thru. The cocky walking is better to watch, but please do not fall into a conclusion that every character should walk like that! Another example, I personally like Darth Vader character in StarWars movie than Luke Skywalker, hell, even Han Solo is charmer than Luke ( but I luuuuuv Leia in bikini... ). Again, do not fall into a conclusion where Bad Guys is charmer. It just how you define a character, visually and personality.
Now, before I finished this, there's one thing to understand : You don't have to apply all these 12 principles in an animation. You have to understand the basic and why. They're just principles, a guideline. What we are trying to achieve is delivering our message to viewer, and you decide how. You can omit one or two or more if you understand and believe it's fine to do so.