mental ray for Maya Shading nodes, such as materials, textures, and lights, allow you to expand the effects achievable when you render with mental ray for Maya.
mental ray for Maya shaders include the Maya Base shaders and Custom Shaders and Phenomena.
Maya comes with a collection of mental ray for Maya base shaders that you can assemble into a wide variety of shading networks. You can use the Maya base shaders exclusively or combine them with the standard Maya (or mental ray) shaders.
You use them the same way you do Maya materials, textures, lights, cameras, and so on. You can achieve sophisticated photorealistic and non-photorealistic looks with them.
For a complete description of Maya base shaders, see mental ray for Maya nodes.