I'm not sure but at a large company i worked for they used the following terms.
intranet - is pretty much you internal netork that is not accessible to the outside world. this may consist of several domains such as usa.microsoft.com, italy.microsoft.com, japan.microsoft.com.
Extranet - emcompasses the outside world. like surfing to
www.duxcw.com from inside of microsoft.
the internet - everything lumped together.
....I think that is how is goes anyway. People at one of my old jobs used these terms I always got confused, because nobody was consistent with their use of these terms. I always thought these were terms computer geeks made up.
Then they disseminated these words into the corporate world and thus were generally reserved for people who didn't know what they were talking about. Now the computer geeks are all kicking back and laughing.
But maybe I'm all wrong and somebody can clarify them.