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SIGGRAPH University : "An Introduction to OpenGL Programming" is an excellent introduction from SIGGRAPH 2013; 3 hours that describe how OpenGL works from some of the people responsible for how it works.
What is interesting, from other sources, is that OpenGL nearly died a few years ago in face of the onslaught of DirectX from Microsoft. Since OpenGL was essentially managed by a volunteer group there wasn't really funding for it's development. Then cell phones appeared, cell phones with enough capability to run a graphics layer, and OpenGL was the go to choice.
I recommend this video to anyone who is interested in what makes computer graphics work.