What is OpenGL - hls333555/OpenGL GitHub Wiki

OpenGL is neither a library, a engine, nor a framework, it is just a specification, there is no implementation in it. Who actually implements those code are GPU manufacturers. The drivers for the GPU such as Nvidia drivers are what contain implementation of OpenGL. Nvidia, AMD, Intel etc., they have their own implementations. Everyone's implementation for OpenGL is slightly different which is why something may work on Nvidia GPU, while the same game may look a little bit different on AMD GPU. As a matter of fact, OpenGL can not be regarded as open source since you cannot see the source code for OpenGL.

The biggest distinction between legacy OpenGL and modern OpenGL is shaders. A shader is a program(code) that runs on the GPU.