Documentation
The reference pages (aka man pages) include complete descriptions of all OpenGL, GLU, and GLX entry points. They cover in detail how to use the commands, all of the parameters you can pass in, and all of the error conditions to avoid.
The GLSL Quick Reference Guide is a one-page PDF providing an instant reference of GLSL's data types, built-in variables and functions.
The OpenGL API and Extension Registry, hosted outside of the SDK, contains the formal Specifications for OpenGL and the OpenGL Shading Language, as well as for companion APIs like GLX and GLU. Specifications are the authoritative documents about how OpenGL is intended to work. They are also the most difficult to read, being written primarily for OpenGL implementers, not developers using OpenGL; so you may find the reference pages combined with tutorials to be a better choice.
The Registry also contains specifications for extensions describing optional features of OpenGL, and header files defining interfaces for extensions. Extension specifications are written as supplements to the core OpenGL Specification. Most new features are first introduced as extensions and only later become part of a core OpenGL version, so using extensions is important to staying current with the evolution of the API.
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