The twelve basic principles of animation, were essentially started by ollie Johnston and Frank Thomas, both were animators at Disney, in their 1981 book The illusion of Life: Disney Animation. The book details not just traditional techniques, like drawing characters moving on a static background, frame by frame. But it details the ideas behind the animations.
Squash and Stretch: Is the the way a soft object or character moves due to an action. For example if a character was bouncing a ball, as the ball hits the ground is condensed in slightly to give an illusion for force being applied to it.
Anticipation: Is a slight movement before a major action to help the viewer anticipate what's about to happen. For example a character about to hit another, would do often an over exaggerated raise of their arm, to insure no viewer misses the action. Also to build interest and emotion the clip.
Staging: Is based on the theatre and that only one major action happens at a time. For example a character falling off a cliff may pause in midair for a second.
Straight ahead action and pose to pose: Is when a scene is drawn frame by frame, start to finish. Pose to pose is more drawing key poses and filling in the major movements later.
Follow through and overlapping: Are the transitions of characters from one action to another, this avoids complete stops. This adds a greater flow to scenes.
Slow in Slow out: Is the acceleration and deceleration of the character throughout a scene. For example the speed between walking and running is often heavily exaggerated to give a better sense of the character really moving fast.
Arcs: Is about circular instead of rigid movement, this gives a better sense of flow and the character being a believable organism. For example if a leg was to move the path the foot moves across from ground> air> ground would be more of a bell-shaped curve instead of squares.
Secondary action: Minor supporting actions to emphasis the main action giving more emotion to the character. For example if a character disagrees with something, they may shake their head, wave their hands and say "no, no, no".
Timing: Is the amount of frames an action has making it last longer or shorter as desired, or as needed.
Exaggeration: Is used in all or most of the techniques and principles to give a more realistic and lively feel to characters. For example a character that falls over make become flat, to emphasis how hard they hit the ground.
Solid drawing: Giving forms an illusion of actually being solid with weight and volume.
Appeal: Essentially what the audience wants to see, classically being heros and villains. Also has to do with character charisma and likable they are.
