About GL_Extension

What are the results of glGetString(GL_Extension)? Are they functions or variables? And where can I obtain information of them?

They are identifiers, each of which refers to a particular OpenGL extension.

The documentation for each extension can be found here -

The string contains names of the extensions that are supported. For information about what each extension does, what constants and functions it use, check the extension registry .

O, I see. Thank you!