From LQWiki

OpenGL (Open Graphics Language) is an API for 3d graphics programming, generally provided with your video drivers. It generally is accessed as a library added to another language, such as C++. Many toolkits, such as GLUT (the GL Utility Toolkit) exist to simplify and extend OpenGL.

Uses

Wine, a linux implementation of the Windows API, uses OpenGL to replace DirectX, a proprietary graphics API distributed by Microsoft.