In a world where things change as fast as the world wide web and the Internet, it is often hard to get a grasp on the exact meanings of buzz-words that suddenly spring up all the time. Some such words appear in newspapers and other publications, are used for a few months and then are never heard again, while others eventually become part of the language we ar all expected to understand.
The word "intranet" seems to have established itself, but it will be a while before you find it in everyday dictionaries. So, what exactly is an intranet?
If you had asked that question in 1995, if you got any answer at all, it would probably be different to the one you would get now. At that time, the word "intranet" would refer to any network within an organisation. Confusingly, if that network was made up of lots of little networks (as is the case for most campus' or large organisations) then that network could be called an "internet" (with a small i, as opposed to the Internet, which is the network of all such networks across the globe).
You can probably see why there is room for confusion on this issue. The question now is what is an "intranet" nowadays?
The best definition I can give is this: An intranet is service making use of the technologies of the world wide web (usually HTML over HTTP) to distribute information within a single organisation over it's internal network. Note that the intranet is no longer the network itself, but a service run over it.
For people with a University computing background, this may sound hautingly familiar. Many Universities have (or had) an intranet for years and never knew it, because it was called a "campus wide information service" or CWIS. Universities all over the country ran CWIS services using a variety of technologies, mostly Gopher or Telnet. As these are both accepted standard Internet protocols, I claim that these are valid intranets, although some would say that only HTTP/HTML technologies count.
Other institutions may have called their local information service a bulletin board, but with the same aim - moving information around within an organisation.
Having established what an intranet is, what are they used for? This is hard to say because of the very nature of an intranet - only the people within the company to which it is specific gets to see it. However, the long and short of it is that whatever information you (or more to the point, the people in charge) want to pass on to everyone else in the company can be put on the intranet.
Intranets are mostly used by large companies, many of whom have had internal networks for years, to cut communications costs. The cases that are often quoted are about the cost of publishing and distributing a health and safety manual or a phone directory within an organisation with a thousand staff spread over thirty sites - with an intranet it is easy. Because the technologies concerned (ie a web browser) can be run on nearly any machine under the sun, an intranet is probably the simplest network technology to install, as well as being one of the easiest to use.
I feel that at this point I should mention the next "big thing" since the invention of the intranet, which is the extranet. Once again, this word would have had a different meaning back in 1995, and it has only been over the last few months that the word has generally been accepted in the networking communities. An extranet is generally defined as an extension of an intranet for the purpose of providing company-specific information to selected other companies or individuals. Examples include post-sales customer support information or product previews to selected distributors.
So, a company might now have three types of information service: (1) It's intranet, for internal information for it's staff, (2) An extranet, for information available to a select few external bodies, and (3) it's world wide web site, for public information. I see it as likely that (3) will be given a new name in a year or so, probably one that already means something slightly different, simply because calling it "our web site" is a bit confusing when the company has three of them.