Clearly, shaders play a crucial creative role in defining the appearance of a CG production. RenderMan has been adopted by many leading studios because it allows special purpose shaders to be added to those that already exist.
Individual shaders are small sub-routines (functions) written in a specialised programming language called the Pixar Shading Language. The language enables new shaders to extend the creative possibilities of the renderer; it allows computer artists to find endless ways of controlling the appearance of a 3D scene through the use of custom shaders. The only limit is their imagination, their ability to write new, or adapt existing shaders and their creative flare at adjusting the parameters that control the visual effect of a shader.
In some respects RenderMan shaders are analogous to plugins for, say, PhotoShop and AfterEffects. Plugins for those applications provide extra functionality to their host program. Likewise, shaders “work” within the environment of a renderer.