What is Cel animation?
Cel animation is also known as Traditional animation, Classical animation, Hand-drawn animation, and 2D animation. It is an animation technique in which the creation of 2D drawings on sheets of transparent plastic is done by hand. These plastic sheets are called cels (from celluloid material they are made of). In Cel animation each cel is crafted after going through a strenuous planning process that involves taking the help of the storyboard and the initial script. The animators create two different types of drawings on the cel.
One side of the cel has an outline of the character and the specific action that’s taking place and the other side is colored by paint (usually acrylic). The colors chosen to fill the drawings are specific to the character and the scene. To create the illusion of the movement of the character, the cels are positioned on a blank background, and then professionals using an Animation Camera capture the sequence. These cels are played at a faster speed to show movement and to achieve the illusion that the characters and the scenes are playing like a movie.
Cel animation requires a professional to have a special set of skills that enables them to pay attention to each and every detail in the scene. This is why this task is usually the responsibility of a group of people so any human errors can be minimized. 
Additional information
Cel animation technique, although expensive, was actually the most widely used method before technological advancement took place and computer animations replaced it. Disney studios last used the complete Cel animation technique back in 1988. Nonetheless, the majority of the 2D animation is still done by hand by a professional animator but the process is sped up now by the help of various types of digital software. This results in cutting the animation costs of the production houses and increases their efficiency.
