OpenGL is a standard API for drawing 3d (and 2d) objects. It was originally developed by Silicon Graphics (SGI) and was based on an earlier in-house library called IRIS GL. The OpenGL standard is now overseen by the OpenGL Architectural Review Board and is available on most modern operating systems including Unix, Windows and Mac.
I've been following the excellent set of tutorials on NeHe's website, and have been doing some experimentation myself. For no obvious reason, I felt the urge to draw a tree, which you can see here.