A cel is a sheet of transparent cellulose acetate used as a medium for painting animation frames. It is transparent so that it can be laid over other cels and/or a painted background, then photographed. (Source: The Complete Animation Course by Chris Patmore.)
Cel animation is incredibly time consuming and requires incredible organization and attention to detail.
Communicating Your Idea
After the idea pops up, a storyboard is created to visually communicate the story to the production team. Then an animatic is created, to see how the film's timing works. Once the story and timing is approved, the artists go to work creating backgrounds and characters that fit "the look" they're going for. At this time, the actors record their lines and animators use the vocal track to synchronize lip movements of the characters. The director then uses the sound track and animatic to work out the timing of the movement, sounds and scenes. The director puts this information on a dope sheet.
Drawing and Painting the Cels
This part of the animation process is the most time-consuming and tedious.
The lead animator makes rough sketches of the key frames (extremes of an action) in a scene. The assistant animator takes those roughs and cleans up the linework, possibly creating some of the in-between drawings. These sheets are passed on to the in-betweener, who draws the rest of the action on separate sheets in order to complete the action established by the animator's key frames. The in-betweener uses the dope sheet to determine how many drawings are needed.
Once the drawings are finished, a pencil test is done to check all that all the movements flow and nothing is missing. A pencil test is essentially a crude animation of the rough drawings.
After the pencil test is approved, a cleanup artist traces the roughs to ensure that the linework is consistent from frame to frame. The cleanup artist's work is then passed on to the inker, who transfers the cleaned-up drawings onto cels before they are given to the paint department to color. If the images are scanned to be used by computers, a lot of the cleanup, inking and painting is done by one person.
Backgrounds of scenes are painted by special background artists. Because the background is seen for longer periods of time, and cover more area than any other single item of animation, they are created with lots of detail and attention to shading, lighting and perspective. The background cels are placed behind the painted action cels in the photographing process (see below).
Filming the Cels
Once all the cels have been inked and painted, they are given to the camera person who photographs the backgrounds, along with their matching cels, according to the instructions on the dope sheet. The processed film, vocal tracks, music and sound tracks are then synchronized and edited together. The final film is sent to the lab to make a film project print or to be put on video. If the studio is employing digital equipment, all these stages happen in the computer before the finished film is output.
As you can see, each step along the way to creating a cel animation requires a lot of work and time, which is usually why shows such as The Simpsons uses teams of people to get the work done.
It also should be noted, if you haven't guessed, that the more frames you create, the more money you spend, either on materials or man hours. That's why shows with low budgets, such as Venture Brothers, repeat backgrounds and frames. Having fewer frames keeps the costs down.