The Principles of Animation

The article titled "The Principles of Animation" published by Annie Gray explains all 28 principles of animation; how they look and why they're important. They act as the basis for all kinds of animation.

Type of Character 

The type of principles used are dependent on the type of character involved in the scene. Living things need to "work and move in a plausible way". Every body part needs to be accounted for, and movements need to be exaggerated for an effective animation.

Primary/Secondary Action

When a character makes a movement, the animator has to take two things into account: primary and secondary movement. Primary movement is a major motion that the entire character's body reacts to, like a walking motion or a character absorbing a punch or catching a flying projectile. A secondary motion is a minor action that one body part or article of clothing that moves in reaction to the primary action. If someone hangs upside down, you would expect their hair to swing back and forth briefly below their head, a secondary action.

Arbitrary Realism

When animating, actions should be exaggerated, but still somewhat realistic and believable. To do this, the animator needs to understand the actual anatomy of whatever they're modeling. To animate a frog, one must know how a frog's bone structure is set up and how it moves around.

It's important to note that the character does not have to look like an animal, but it must move like an animal.

http://www.animationarena.com/principles-of-animation-2.html