Cell Shading/Animation

Cel Animation

Cel animation is a traditional 2-dimentional process of animation or artwork where characters and moving objects are drawn on cels (clear sheets of celluloid or more common cellulose acetate) and placed over a still painted background.  The Characters are traditionally outlined with black ink by hand (or more recently, xerographed on) and then colored in (traditionally with acrylic paint or in modern times digitally or with an animation photo transfer process).  The traditional paint-on color was done on the reverse side of the cell to prevent the brush strokes from being visible.  It was commonly used by Disney, Universal Pictures, and other 2-D animation studios in the beginning, in classics such as Peter Pan.

 

and though most of the traditional media and slow work has now been abandoned for the faster, more efficient digital medium, the idea of the technique itself is still used today in films like the Princess and the Frog, just with the more modern method combined with the original hand drawings.

This is a diagram of how it was done traditionally:

The APT process

(Animation Photo Transfer) process, used in films like the Black Cauldron and The Great Mouse Detective, is a process that involves photographically transferring  the lines and color blocks onto the acetate sheets (cels) using a certain UV sensitive ink.  This process first made it possible to make the lines color instead of black, though you can do that with xerographing the image now as well. 

The traditional technique is also still commonly used in things like Japanese animation.

 

If the pictures don't show up, sorry.  My technical skills are still in progress.