How does one define Shader in CG Design & what is Shading? Is it only about how light

[shippo113]

I am trying to learn OpenGL and wondering about this.
How does one define Shader in CG Design & what is Shading? Is it only about how light is reflected off a surface?
I understand that shades tell us about where light source is and where the shadow is, but how does one define a Shader?

[Alfonse Reinheart]

What is "CG design"?

As for what a shader is, in terms of modern consumer graphics cards, a shader is a program that is executed on the GPU, which computes some part of the graphics pipeline.