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
• Anticipation
• Staging
• Straight Ahead vs Pose to Pose
• Follow Through and Overlap
• Slow In and Slow Out aka easing
• Arcs
• Secondary Action
• Timing
• Exaggeration
• Solid Forms
• Appeal

These are a great starting point for UI and motion designers as well as Illustrators an animators.