During the dawn of the empire that would later become Walt Disney Studios, there existed a group of nine animators. Though they were only middle-aged when hired, these men were eventually known as Disney's Nine Old Men. It was these nine men who created Disney's iconic animation style, imbuing sketches and animation cells with the stuff of life and wonder.
Over the years, the Nine Old Men developed 12 basic principles of Disney animation. In addition to Disney animators, artists all over the world took note. The Nine Old Men have all passed away now, but their principles live on, thanks in part to two of their members getting together back in 1981 and publishing a book on the group's traditions. The book was called Disney Animation: The Illusion of Life, written by Ollie Johnston and Frank Thomas.
The work outlined weight and speed-lending techniques like the "squash and stretch" we see when a character slams into a closed door. Another rule, "follow through and overlap," dictates that the same character should jostle about as its body deals with the sudden stop of being struck with the door.
While a wonderful work, the book can't actually show you what Disney's techniques were capable of creating. For that you'd have to take a pencil in hand and try the 12 principles out for yourself. (Or you can just watch the video below.) Of course today, animation is done on computers and the two-dimensional cartoons of yesteryear have been brushed aside to make way for 3D. Still the 12 principles remain the same: immutable tricks of the trade that help bring life to colored shapes.