OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Shaders and OpenGL

The modern OpenGL rendering pipeline relies very heavily on using shaders to process the data you pass to it. About the only rendering you can do with OpenGL without shaders is clearing a window, which should give you a feel for how important they are when using OpenGL. Versions of OpenGL before (and including) Version 3.0, or those using a compatibility-profile context, include a fixed-function pipeline that processes geometric and pixel data for you, without shaders. Starting with Version 3.1, the fixed-function pipeline was removed from the core profile, and shaders became mandatory.

Shaders, whether for OpenGL or any other graphics API, are usually written in a specialized programming language. For OpenGL, we use GLSL, ...