There is a tutorial about Cel Shading at Gamedev.net:
here is a little bit:
Cel-Shading is the "art" of rendering your objects to look like cartoons. Various cartoon effects can be achieved with very little modifications to the original source code. You can have anime effects, as seen in DragonballZ or Gundam, or "classic" cartoons such as Loony Toons. Cel-Shading is a very powerful form of rendering, and it's results can completely change the "feel" of a game. For instance, the cartoon-like graphics of Jet Set and Grind Radio on the Dreamcast add to the atmosphere and help create one hell of a funky game. However, don't expect this form of rendering to make your game amazing overnight - just play Loony Toons Space Race and you'll see that sometimes it doesn't really help at all.