Contrary to popular belief, this is not simply a misspelling of "Internet." "Intra" means "internal" or "within," so an Intranet is an internal or private network that can only be accessed within the confines of a company, university, or organization. "Inter" means "between or among," hence the difference between the Internet and an Intranet.
Up until the last few years, most corporations used local networks composed of expensive proprietary hardware and software for their internal communications. Now, using simple Internet technology, intranets have made internal communication much easier and less expensive. Intranets use a TCP/IP connection and support Web browsing, just like a typical Internet connection does. The difference is that Web sites served within the intranet can only be accessed by computers connected through the local network. Now that you know the difference between the Internet and an intranet, you can go around telling people on the street what you know and impress them.