| Renderman (full name: Photorealistic Renderman) is Pixar's flagship 3D rendering tool, marketed primarily to the broadcast and film industries. Originally developed by Pat Hanrahan and Bill Reeves in 1988, Renderman was based on the REYES algorithm, which is patented by Pixar (and is probably a significant factor in the domination of the field by the company). Pixar won an Oscar in 1993 for Renderman.
Renderman is both a product and a standard. The Renderman standard describes a programming API, a binary file format, and a text file format. These are essentially scene description files. 3rd party vendors are free to implement the Renderman standard and use certain Renderman trademarks with their products if they meet criteria outlined by Pixar. Blue Moon Rendering Tools is a Renderman-compliant raytracer, for instance. While there are a few software packages on the market that are Renderman-compliant, the standard is unlikely to become a widely adopted one. The market that Pixar competes in is small enough that there is not a broad need for interoperability beyond conversion tools. The Renderman standard is somewhat tailored to the nature of the rendering algorithm and Pixar's design goals, and since they have patents on their algorithms, the need for a common interface to REYES-based products isn't there.
A comon misconception about Renderman is that it is a raytracing-based product. The REYES algorithm is a scanline algorithm whose design goals were speed and flexible, programmatic surface behavior modeling. Key features of raytracing that are missing or simulated in Renderman are reflections, refractions, and shadows. There are tools available, however, to use BMRT or other raytracing-based Renderman-compliant renderers in conjunction with the Renderman product, raytracing only those parts of the scene which require the added features of raytracing. Pixar has used these tools before; the glass jar in the bug city in A Bug's Life was rendered using this method.
Patents are not the only reason Pixar has dominated film rendering products. Extensive R&D has added cutting edge features like subdivision surfaces (highlighting this feature was the raison d'etre for the short film Geri's Game), hair and feather modeling, and extensive surface modeling. The high visibility of well-used Pixar products in hugely successful films has also helped cement their reputation as the number one player in their market.
(hope that doesn't read too much like a press release -- I just have a lot of respect for what the company has accomplished.) | 
