What is OpenGL?
In this lesson, you will learn what OpenGL is and how it enables you to program in 3D.
What exactly is OpenGL? It's a way to draw stuff in 3D. It can also be used for 2D drawing, but this site doesn't focus on that. There are better tools for straight 2D drawing, such as SDL and Allegro.
The graphics card is where the 3D computation happens. The purpose of OpenGL is to communicate with the graphics card about your 3D scene.
So why not talk to the graphics card directly? Each graphics card is a little different. In a sense, they all speak different "languages". To talk to them all, you can either learn all of their languages, or find a "translator" that knows all of their languages and talk to the translator, so that you only have to know one language. OpenGL serves as a "translator" for graphics cards.
Next is "Part 0: Getting OpenGL Set Up".