The 12 Basic Principles of Animation

1. Squash and stretch
2. Anticipation
3. Staging
4. Straight Ahead Action and Pose to Pose
5. Follow Through and Overlapping Action
6. Slow In and Slow Out
7. Arcs
8. Secondary Action
9. Timing
10. Exaggeration
11. Solid Drawing
12. Appeal

We could also add theses two, since they are very important in animation:
13. Mass and weight
14. Character acting


ILLUSION OF LIFE, THE: DISNEY ANIMATION
by Frank Thomas & Ollie Johnston.