Shaders are descriptions of surface materials or effects that control what a surface looks like when it is rendered (for example, color, reflectivity, roughness).
In order for a surface to have the properties of a shader, you must assign the shader to the surface. You can assign a single shader to several different surfaces. You can also layer several shaders onto a single surface. By default, all surfaces have the DefaultShader assigned to them.
Shaders have literally hundreds of parameters that let you simulate virtually any material you can imagine. But this infinite variety is based on two basic decisions: what shading model to use, and how to set or apply textures to the parameters of that shading model.