Textures are two-dimensional or three-dimensional patterns which you can assign, or map, to certain parameters of environments, shaders, and lights. You can also assign textures to certain parameters of other textures.
There are three basic types of textures:
The Texture Procedures window contains a list of all textures (Surface, Environments, and Solid) and a list of all Texturable Values for the active environment/shader/texture/light.