OpenGL is the premier environment for developing portable, interactive 2D and 3D graphics applications. Since its introduction in 1992, OpenGL has become the industry's most widely used and supported 2D and 3D graphics application programming interface (API), bringing thousands of applications to a wide variety of computer platforms. OpenGL fosters innovation and speeds application development by incorporating a broad set of rendering, texture mapping, special effects, and other powerful visualization functions. Developers can leverage the power of OpenGL across all popular desktop and workstation platforms, ensuring wide application deployment.
The OpenGL Red Book is a good beginners guide to all of the basic features of OpenGL.
MSDN's OpenGL Reference is a good, comprehensive list of the API. However it only contains documentation up to 1.1.
For an API reference beyond 1.1, you can read the Extension registry. However as it is the actual specification rather than user documentation it is not the most user friendly to read. The Blue Book provides a reference for later versions and some common extensions.