In mental ray, a shader
is a function that calculates light effects. There can be shaders
for lights, cameras (lens shaders), materials, shadows, and so on.
In 3ds Max Design, the mental
ray translator provides the functionality of light and camera shaders.
Material shaders correspond to 3ds Max Design materials.
A number of shaders are
provided with 3ds Max Design. See
mental ray Shaders,
mental ray Connection Rollout,
mental ray Materials.
The mental ray manual, Programming
mental ray, describes how to write custom shaders.