After launching the application, select one of the following tab:
OpenGL
Extensions
Display Modes
Pixel Format
Report
Test
Registry
Database
Credits

OpenGL
Display system information and core feature test.

Extensions
Display extensions, sorted by vendor. If you have an Internet connection, you can retrieve from OpenGL.org the extensions's specifications that explain the available extensions. You can click on extension and press 'Search' for getting extension's specification from severals different databases. You can also use the left button mouse.

Display Modes
Display all supported display modes.

Pixel Formats
Display all supported pixel format
Report
Create a report based from current configuration.

Report
The OpenGL diagnostic tool performs some OpenGL initialisation and graphical test. It will use the desktop resolution. So in case of slow downs or initialisation errors, use a smaller desktop resolution.

OpenGL 1.1 : use the standard OpenGL without extension usage.
OpenGL 1.2 : OpenGL test using GL_lock_array and glDrawElement.
OpenGL 1.3 : OpenGL using GL_lock_array and multitexturing.
OpenGL 1.4 : OpenGL test using GL_lock_array and multitexturing using a DOT3 shading and an ARB vertex program code.
OpenGL 1.5 : OpenGL test using Vertex Buffer Object.
OpenGL 2.0 : OpenGL test using a GLSL 1.0 program.
OpenGL 2.1 : OpenGL test using pixel buffer object.

Registry
(Windows only). Displays registry keys to enable 'hidden' extensions.
Database
Display database of all known vendors, renderers and extensions.

Credits
execom / main code.
arekkusu / cocoa code.
genevois / web site / graphist.

Links
Here a list of useful links, related to OpenGL Extensions and Programming:
Apple' s OpenGL Extensions Guide
OpenGL® Extension Registry
Delphi3D PC OpenGL Hardware Registry
Understanding and Detecting OpenGL Functionality
MacOS X OpenGL information