The Principles outlined here were taken from John Lasseter’s “Principles of Traditional Animation Applied to 3D Computer Animation” July 1987 (SIGGRAPH 87). The original page can be found here.

Many of the principles of traditional animation were developed in the 1930’s at the Walt Disney studios. These principles were developed to make animation, especially character animation, more realistic and entertaining. These principles can and should be applied to 3D computer animation.

The following principles were developed and named:

1. Squash and Stretch - defining the rigidity and mass of an object by distorting its shape during an action.

A classic example is a bouncing ball, that squashes when it hits the ground and stretches just before and after. The stretching, while not realistic, makes the ball appear to be moving faster right before and after it hits the ground.
These deformations are very important in facial animation, since they show the flexibility of the skin and muscle and also the relationship between the different facial parts. In very early animation, a character chewing something only moved its mouth and it appeared unrealistic. A later innovation was to have the entire face moving with the mouth motion, thus looking more realistic. This can be exaggerated for effect. A broad smile or frown similarly involves more than the mouth.

2. Timing and Motion - spacing actions to define the weight and size of objects and the personality of characters.

Timing can also affect the perception of mass of an object. A heavier object takes a greater force and a longer time to accelerate and decelerate. For example, if a character picks up a heavy object, e.g., a bowlng ball, they should do it much slower than picking up a light object such as a basketball. Similarly, timing affects the perception of object size. A larger object moves more slowly than a smaller object and has greater inertia. These effects are done not by changing the poses, but by varying the spaces or time (number of frames) between poses.

3. Anticipation - the preparation for an action.

Anticipation can be the anatomical preparation for the action, e.g., retracting a foot before kicking a ball. It can also be a device to attract the viewer’s attention to the proper screen area and to prepare them for the action, e.g., raising the arms and staring at something before picking it up, or staring off-screen at something and then reacting to it before the action moves on-screen. An example of this is the opening scene of Luxo, jr.. The father is looking off-screen and then reacts to something. This sets up the viewers to look at that part of the screen so they are prepared when Luxo, jr. hops in from off-screen.
Anticipation can also create the perception of weight or mass, e.g., a heavy person might put their arms on a chair before they rise, whereas a smaller person might just stand up.

4. Staging - presenting an idea so that it is unmistakably clear. An important objective of staging is to lead the viewers eye to where the action will occur so that they do not miss anything. This means that only one idea at a time occur, or else the viewers may be looking at the wrong thing. So, the main object should be contrasted in some way with the rest of the scene. A good example is motion, since the eye is drawn to motion in an otherwise still scene. In a scene with everything moving, the eye is drawn to a still object.

5. Follow Through and Overlapping Action - the termination of an action and establishing its relationship to the next action.

An example is in throwing a ball - the hand continues to move after the ball is released. In the movement of a complex object different parts of the object move at different times and different rates. For example, in walking, the hip leads, followed by the leg and then the foot. As the lead part stops, the lagging parts continue in motion.
Heavier parts lag farther and stop slower. An example is in the antennae of an insect - they will lag behind and them move quickly to indicate the lighter mass.

6. Straight Ahead Action and Pose-to-Pose Action - The two contrasting approaches to the creation of movement.

Straight Ahead Action in hand drawn animation is when the animator starts at the first drawing in a scene and then draws all of the subsequent frames until he reaches the end of the scene. This creates very spontaneous and zany looking animation and is used for wild, scrambling action.
Pose-to-Pose Action is when the animator carefully plans out the animation, draws a sequence of poses, i.e., the initial, some in-between, and the final poses and then draws all the in-between frames (or another artist or the computer draws the inbetween frames). This is used when the scene requires more thought and the poses and timing are important.

7. Slow In and Out - the spacing of the in-between frames to achieve subtlety of timing and movement.

This is usually achieved by using splines to control the path of an object. The various spline parameters can be adjusted to give the required effect. In 3D Studio this is controlled by the parameters Ease To and Ease From in the Key info window (from the Track info window). When these are zero, there is a constant velocity in either direction, i.e., to/from the keyframe. When Ease To is set to a higher value, the motion is faster as it leaves the previous keyframe and slows as it approaches the current keyframe. When Ease From is set to a higher value the motion is slower leaving the current keyframe and speeds up as it approaches the next keyframe. The tick mark spacing shows the velocity with closer tick marks indicating a slower rate and spaced out ones indicating a faster rate.

8. Arcs - the visual path of action for natural movement.

9. Exaggeration - Accentuating the essence of an idea via the design and the action. Exaggeration does not mean just distorting the actions or objects arbitrarily, but the animator must carefully choose which properties to exaggerate. If only one thing is exaggerated then it may stand out too much. If everything is exaggerated, then the entire scene may appear too unrealistic.

10. Secondary Action - the action of an object resulting from another action. It can be used to increase the complexity and interest in a scene. It should always be subordinate to and not compete with the primary action in the scene. An example might be the facial expression on a character. The body would be expressing the primary action while the expression adds to it. Another example would be the trailing electrical cord of Luxo, jr.

11. Appeal - creating a design or an action that the audience enjoys watching. This is equivalent to charisma in a live actor. A scene or character should not be too simple (boring!) or too complex (can’t understand it). One principle to achieve this is to avoid mirror symmetry. Asymmetry tends to be more interesting and appealing.

Personality in character animation is the goal of all of the above. The different principles should be applied in a fashion to produce a consistent personality. This means that the animator must have a good idea of the desired personality before beginning the animation.

Please follow the link to the original source for more information.