The twelve principles of animation were created by Disney animators Ollie Johnston and Frank Thomas in 1981.
The principles that they describe are based on the work of Disney animators from the 1930s onwards, the aim was to create realistic and fluid animation that obeys physics and follows scenarios and reactions in a way that is true to life.

The twelve principles include; squash and stretch, anticipation, staging, straight ahead action and pose to pose, follow through and overlapping action, slow in and slow out, arc, timing, exaggeration, solid drawing and appeal.

In short, squash and stretch refers to the realistic way objects fall and move. The idea is to convey both weight and flexibility and impact into the animation. This helps greatly in making the animation seem less rigid and robotic and more lifelike.

Anticipation refers to how characters and objects move. Instead of the character lifting their arms and just throwing the ball downward, animators will take care to make the knees straighten before the throw and bend after it. Other body parts may be altered as well to give a realistic appearance.

Staging makes it obvious what the most important subject or object is in the scene either through lighting or composition.

Straight ahead action and pose to pose are different ways to animate. Straight ahead action scenes are drawn frame by frame, creating a very fluid animation. The drawback however is that proportion is hard to maintain. Pose to pose needs only a few key frames to be drawn then later you fill them in, this works better on scenes with a focus on surroundings or composition.

Follow through and overlapping action refers to realistic movements within animation, making it seem like the laws of physics apply. “Follow through” means that body parts such as hair or maybe arms should continue movements after character has stopped but be subsequently pulled back to create realistic movement. “Overlapping action” means that body parts should move at different times and speeds.

Slow in and slow out is all about speed and how things slow before stopping or when starting.

An animation that follows movement with implied arcs achieves greater realism. For example, the faster an object goes the broader and flatter its arc will become.

The addition of secondary actions gives the scene a more realistic, fluid and lifelike appearance. These secondary movements will always emphasise the main movement e.g. arms swinging while walking.

Timing refers to the number of drawing or frames given in a sequence or action, translating into speed of action in the animation.

Exaggeration prevents the animation from looking dull and too realistic. It brings out comedy, drama and beauty and creates a more visually interesting image.

Solid drawing means taking into account three dimensional space, making sure objects do not see, flat or weightless.

Appeal in animation is what makes characters likeable or at least enjoyable to watch. Baby-ish and attractive faces create more likeable characters while hard to read and ugly faces tend to do the opposite