OpenGL extensions


I’m a little confused about OpenGL’s extensions, so maybe you can help me. When using a normal extensions under Windows, you have to call the GetProcAdress function to register the extension function, right? But what about those extensions, that have already been made part of the OpenGL specification? Can I just call the functions then like any other GL function, or is it still necescary to treat them as extensions? Surely I could do that since OpenGL is backwards compatible with earlier versions, but what would be the “correct” thing to do?
And how do I ensure, that my version of OpenGL is the latest? Do I have to download new GL.h files or what? I’m currently using NVidias drivers, but they do not seem to include any .h files.
Can someone offer some clarifications on this?

AFAIK, at least on windows you have to use GetProcAddress even for extensions that are incorporated in OpenGL version 1.2, 1.3 or whatever. That is because the opengl32.dll that is supplied by Micros**t only supports the 1.1 functions directly.
All other functions are exposed by the driver manufacturers and can be obtained through GetProcAddress.