Materials define the surface properties of scene objects. They are stored in the BaseDocument. Shaders are used to define these properties dynamically.

BaseMaterial is the base class for all materials in Cinema 4D. See BaseMaterial Manual.
Material is the class representing a Cinema 4D standard material. See Material Manual.
NodeMaterial is the class representing a BaseMaterial containing a NodeSystem. See NodeMaterial Manual.
BaseShader is the base class for all shaders in Cinema 4D. See BaseShader Manual.
To prepare a material or shader for sampling an InitRenderStruct element is needed. See InitRenderStruct Manual.

A material can be assigned to one or multiple objects. This connection is defined with a tag:

A TextureTag is used to assign a material to an object. See TextureTag Manual.

When an object is shaded its normals and UV coordinates are important. This information is stored in tags:

A UVWTag stores UVW coordinates for a polygon object. See UVWTag Manual.
A NormalTag stores custom normal vectors for a polygon object. See NormalTag Manual.

The node based material utilizes so called core nodes. It is possible to create custom core nodes:

Core Nodes Implementation