One of the greatest strengths of OpenGL is that it was designed to be readily extensible to accomodate new hardware innovations.
Using the OpenGL extension mechanism, hardware developers can differentiate their products and incorporate new features by developing extensions that allow software developers to access additional performance and technological innovations. These extensions provide OpenGL application developers with new rendering features above and beyond the features specified in the official OpenGL standard. OpenGL extensions keep the OpenGL API current with the latest innovations in graphics hardware and rendering algorithms.
In order not to compromise the readability of the core specification, ARB extensions are not integrated into the core language; instead, they are made available online in the OpenGL Extension Registry (as are a much larger number of vendor specific extensions, as well as extensions to GLX and WGL). Extensions are documented as changes to the Specification.
The OpenGL Extension Registry is maintained by the OpenGL ARB and contains specifications for all known extensions, written as modifications to the appropriate specification documents. The registry also defines naming conventions, guidelines for creating new extensions and writing suitable extension specifications, and other related documentation.