The 12 principals of animation are a set of guidelines created by Disney animators Ollie Johnston and Frank Thomas doing the 1930s.
This video does a great job of explaining all 12.
- Squash and Stretch
- Straight Ahead vs Pose to Pose
- Follow Through and Overlap
- Slow In and Slow Out aka easing
- Secondary Action
- Solid Forms
These are a great starting point for UI and motion designers as well as Illustrators an animators.