Animation is a method by which still figures are manipulated to appear as moving images. In traditional animation, images are drawn or painted by hand on transparent celluloid sheets to be photographed and exhibited on film. Today, many animations are made with computer-generated imagery (CGI). Computer animation can be very detailed 3D animation, while 2D computer animation (which may have the look of traditional animation) can be used for stylistic reasons, low bandwidth, or faster real-time renderings. Other common animation methods apply a stop motion technique to two- and three-dimensional objects like paper cutouts, puppets, or clay figures.
Squash and stretch is used to give characters and objects the illusion of weight, gravity, flexibility, etc. Think of a bouncing ball; the ball gets stretches longer and thinner as it moves up and down and squashes flat when it hits the ground.
Anticipation prepares the viewer for what is about to happen, and also helps make the animation more realistic and energetic. A character cannot jump without first bending their knees, or throw a ball without first drawing their arm back, for example.
Staging is the animation version of composition in illustration. The animator must stage the action so it is easy to follow, using motion to guide the viewer's eyes to the important parts on the screen. Non important movement should be kept to a minimum to avoid distraction and a cluttered composition.
There are two ways to approach drawing animation; straight ahead and pose-to-pose. Straight ahead means drawing each frame in order from start to finish, whereas pose-to-pose involves laying out the starting pose, ending pose, and all important poses between, and then filling in the rest. Straight ahead generally provides more fluid, realistic motion, while pose-to-pose makes it easier to create impactful and dramatic poses and moments.
Follow through and overlapping action are essential components to making realistic and compelling animations. Follow through refers to how not every part of an object or character will move or stop at the same time or rate, for example, a super hero with a cape will have his cape blow behind him when he flies and sweep forward when he stops. Overlapping action refers to different parts of an object or character moving in different directions or speeds at once, or in other words, overlapping. For example, when a character runs, their arms will be moving in opposite directions, and their arms and their head will not move at the same time or rate.
Slow in and slow out is likely the easiest to understand. A car cannot go from 0 to 60 instantly, nor vice versa. It requires time to speed up and slow down, and similarly animated characters need to ease in and out of their motions to seem realistic and lifelike.
Most objects move in arcs, like a ball tosses in the air. Likewise, having characters or objects move in arcs in your animations will preserve the realism as well as make things interesting. Moving in a straight line from point A to B isn't that interesting.
Secondary action helps make your animations more interesting and reactive, for example a character's clothes or hair moving in the wind as they run, or a character's face reacting to another character's action.
Timing is perhaps the hardest concept to wrap one's head around, as it is mostly intuitive. Timing can change the feeling of an animation, as well as sell it as realistic. For example, a character throwing something. If the timing is incorrect, it can look unrealistic, as if they did not put enough force into the throw to send it as far as it goes. Proper timing can help sell realism, as well as convey emotion. For example, a soft and smooth movement is calm, and a fast whip-like throw is angry.
Too much adherance to realism can be very boring, so add some exaggeration to make things more impactful and interesting! If your character is jumping, don't just bend their knees, get that squatting low with their face almost to the ground, and then have them rocket into the sky!