Cel Shading is the technique of rendering 3D models and scenes so that they look "hand drawn" like a Disney movie or "Ren and Stimpy" "The Simpsons" etc... Basically you use high tech computers and software to make something look like it was drawn with a pencil.
This is very easy to achieve in Cinema 4D XL, you just simply check a few boxes in your render settings and you're done. There are some things to consider when setting up your scenes and building your models, but first a look at the different effects and how to achieve them.
You activate the cel shading under the pop up menu - File/Preferences/Render. Set the render mode to "Surface colour" or "Surface B/W"
This turns off the raytracer and of course transparency and reflections.
Then change the pop up that says "General" to "Options"
Here you have two choices that affect cel shaded output.
"Black Edges" If this is checked all models will have a black outline similar to pencil lines.
"No Light" If this is checked you will not get shadows, or shading on the darker side of objects. Basically, you will only see the outline of your object
Here's "Plungo" raytraced with full reflections and shadows
Surface mode settings are ignored by the raytracer
And here are all the cel shaded variations.
In "Surface B/W" mode the "No Light" setting has no effect
You can take these pictures into Photoshop or After Effects and get some great effects by layering them together with different transparency settings and transfer modes.
You will need to add more detail for cel shaded projects as only geometry edges get black lines and if you select "No Light" you only see the profile of the model and no other detail.
For example, the ball in the pictures above is two separate parts, one part is the red checkers, one is the blue checkers. That way I still get the checkerboard pattern in the cel shade versions.
In this example you can see what will happen if you don't have your geometry as separate objects and use the No Light setting.
In this picture all the face parts are separate objects and all render with nice lines around them. 
In this picture everything is one solid object.
Not very pretty.
Cel shading can be a very expressive technique and it's really easy to use. Play with it and have fun!
