OpenGL is a software system for computer graphics rendering. It is a de facto international standard, supported by many companies, and originated by Silicon Graphics. There are some slides that outline its basic ideas, and some simple examples.
This page points to some OpenGL resources.
There is much additional information for local users.
The OpenGL WWW Centre provides general information as well as links to many other OpenGL resources, including documentation.
SGI's Doc & Technical Information page has many useful links, including a number of specification documents.
The Graphics Library Utility Toolkit (GLUT) provides information about a system that provides a simple window management and interactive toolkit. OpenGL is purely for rendering; in itself it provides no mechanism for creating windows and for interaction.
The OpenGL org pages provide more commercial information, for example, if you're looking for an OpenGL to run on a particular system.
There is an OpenGL look-alike system called Mesa. This is worth getting if you have no other access to an 'official' OpenGL implementation. There is a commercial OpenGL for MacOS, but there is also a Mesa implementation.
GLX is an extension to X11 Windows that is required for running OpenGL in this environment.