Differences between internet,intranet and extranet

How does one distinguish between an intranet and an extranet? When I access company-only information over the Internet, is that an intranet or an extranet?
A: The term "intranet" is somewhat misleading conceptually, because it invites a contrast to the term "Internet." The real contrast is with the World Wide Web--an important distinction, because "Internet" focuses on physical and technical networks, while the Web focuses on the set of content accessible on that physical and technical infrastructure.
When I coined the term "IntraNet" at Amdahl Corp. in the summer of 1994, it did have the connotation of an internal Web rather than just an internal Internet. In fact, the term we used internally before this was the too-cumbersome "Enterprise-Wide Web." So, while the ambiguity of "intranet" was apparent even back then, for lack of a better alternative, it caught on.
In the early days, I defined an intranet as "An infrastructure based on Internet standards and technologies that supports sharing of content within a limited and well-defined group." The "infrastructure" referred to the organizational and management infrastructure that created, managed, and shared the content. The only technical constraint was that the physical network be based on the Internetworking Protocol (IP).
You might notice that this definition encompasses what we call extranets today, because the defining factor is a "limited and well-defined group," and does not specify any official organizational affiliation. The Web, in contrast, is an unlimited group.
Today I think of intranets, extranets, and the Web as collections of content. An intranet is a set of content shared by a well-defined group within a single organization. An extranet is a set of content shared by a well-defined group, but one that crosses enterprise boundaries.
These access distinctions are important, because Web-based content uses the same technical infrastructure regardless of access decisions. This means it is much easier to change access to specific content than it was in the old proprietary world,where making something more widely available often entailed a major conversion effort.
As technical infrastructure becomes less of a barrier to accessing specific content, it becomes important to pay attention to how, or if, we want to restrict access. The terms "intranet" and "extranet," as imperfect as they are, provide us with conceptual and pragmatic tools for discussing to whom we want to make specific content available.
These terms may continue to evolve in meaning. For now, a set of content accessed by members of a single organization is an intranet, even if the information travels across the public Internet infrastructure.