They are header files, not libraries. A header file is part of some library (or application).
What do those files contain? Do they have any comments?
Do you use a package management system that could tell, which packages provide those files? Packages tend to have description.
The gl.h is one header from OpenGL library. There are more than one OpenGL library version, because each hardware vendor tends to add extensions that their GPU's support.
If you misunderstood my question, I will restate my question.
I've seen people using gl and glew but not together...which brings me a question, I know that both library is OpenGL library but what's the difference between these two? Why do some people prefer gl over glew and glew over gl?