The RenderMan Interface Specification, or RISpec in short, is an open API developed by Pixar Animation Studios to describe three-dimensional scenes and turn them into digital photorealistic images. It includes the RenderMan Shading Language.
As Pixar's technical specification for a standard communications protocol (or interface) between modeling programs and rendering programs capable of producing photorealistic-quality images, RISpec is a similar concept to PostScript but for describing 3D scenes rather than 2D page layouts. Thus, modelling programs which understand the RenderMan Interface protocol can send data to rendering software which implements the RenderMan Interface, without caring what rendering algorithms are utilized by the latter.
The interface was first published in 1988 (version 3.0) and was designed to be sufficiently future proof to encompass advances in technology for a significant number of years. The current revision is 3.2.1, released in November 2005.
Read more about RenderMan Interface Specification: RenderMan Interface Specification, Language Bindings, Further Reading