From BlenderWiki

Material Node Types

This section is organized by type of node, which are grouped based on similar functions:

• Input - Introduces a material or component to the node map.
• Output - Displays the result in progress as a small image.
• Color - Manipulates the colors of the material.
• Vector - Change the way light is reflected off the material.
• Convertors - Convert colors to other material colors.
• Groups - User-defined groups of nodes.
• Dynamic - Custom nodes defined by Python. These are also known as PyNodes.