In a nutshell, Renderman is a specification between a model system and a rendering system. In the nearest equivalent to PC gaming technology, think of it as the shading architecture for PC graphics cards that allow per fragment lighting. There are similar systems out there that may help you get a grasp on it (though nowhere near as complex). NVidia's Cg is one that also comes somewhat close.
Though for once I'd like to see Pixar break from their existing mold and do something completely different. They're unchained from Disney now, and I'd like to see them break off and do other types of cinema than just cartoon-comedy.
it wouldtrock if pixar makes a 2d studio then they can prove to disney that talent makes a good movie not graphics. im still bitter that Disney ended all thier 2d animation and forced every one to learn 3d. they truley are killing the art form 2d is where it started and it should stay its foundation.