OpenGL is an API which can run on most platforms including Windows, Linux, MacOs, iphone, Nintendo Wii, some windows mobile devices, and many many others.
I will add tutorials soon, in the mean time, look at http://nehe.gamedev.net/ for a comprehensive list of tutorials. If you plan to try to develop for multiple platforms, I reccomend looking into SDL, or NeHe's tutorials with SDL. SDL gives your programs the ability to interface with many different systems
Instead of having to go through the process of creating a window in windows, and a lot of other mundane tasks, as well as modifying programs to run under other system, SDL does it for you.
