OpenGL is a graphics standard and API which targets the Desktop, workstation and mobile markets. It is designed to be easy to accelerate with dedicated computer hardware, and hence most implementations give greatly improved performance over traditional software rendering. OpenGL is used for applications like CAD software and computer games. It is also cross-platform and with bindings for several programming languages.
OpenGL (or Open Graphics Library) is a cross-language, multi-platform application programming interface (API) specification for 2D and 3D graphics rendering. The API is typically used to interact with a graphics processing unit (GPU), to achieve hardware-accelerated rendering.
Silicon Graphics Inc. (SGI) started developing OpenGL circa 1991 and released the first implementation in 1992. The specification is currently at version 4.5 (August 2014). Applications use it extensively in the fields of CAD, virtual reality, scientific and data visualization, flight simulation and video games. OpenGL is now managed by the non-profit technology consortium Khronos Group.
OpenGL.org, the home of OpenGL.
The Wikipedia page on OpenGL.
Introduction and tutorials at OpenGLBook.com.
Another collection of tutorials targeting all the major OSs.
Questions including code that uses the OpenGL API should use this tag. This not only applies to the C version of the API, but to any language that has bindings to OpenGL.
For questions that also include GLSL shader code, add the glsl tag as well.