The Pixar Look Libraries contain a collection of different types of Looks, pre-made shaders. These libraries are available separately on the Pixar Looks CD.  

The collection of pre-fab Looks was made less important with the introduction of Slim. Slim provides a quick and easy interface for generating custom RenderMan Shaders without programming, however if there isn't the time for Slim, this collection can be useful. If you happen to have the old looks, they will generally work with the renderer.