A tag is a word or short phrase attached as metadata to an object on a computer used to make that object easier to re-find by the person who tagged it, or more likely to be discovered by others looking for new objects. An object might typically be a Web page, a photograph, information about a book, etc. A prototypical tag expresses what the object is about and is made public.
Tags require tagging systems that enable users to create the tags and that provide ways for users to find all the tagged objects by searching for a tag. For example, at Flickr, users can post a photograph that then can be tagged by other users. Search for the tag "hawaii" at Flickr, and the site will show you the thousands of photos that users have tagged that way.
Within the realm of computers, tags were preceded by keywords. For example, Microsoft Word allows users to keyword a document with words that will make it easier for them to find those documents later on. Keywords may or may not themselves appear in the document. For example, you might keyword your Word document on Alaskan Fishing with the word "sports," even if that word doesn't show up in your article.
Tags are like keywords in that they're linquistic metadata attached to an object. But tagging as it's now understood adds some properties apparent at the site often credited with kicking off the current interest: del.icio.us (Delicious.com also works.) There users can create lists of Web pages they want to remember, each tagged with as many tags as they'd like. So far, tags are operating like keywords. But, del.icio.us makes the lists of pages and their tags public, so that if you search for the tag, say, "sf," you'll be shown a list of every page anyone at del.icio.us has tagged that way.
Many sites that enable tagging also allow users to subscribe to a tag stream. For example, if you have a particular interest in curries, the site would allow you to subscribe to that tag so that you could check your aggregator or perhaps your email inbox in which the latest objects tagged "curry" would be listed. This enables a distributed group to share their discoveries.
Some sites aggregate tagged objects. For example, search forthe tag "Iraq" at Technorati and you'll be shown a page that lists the Web pages tagged that way at del.icio.us as well as at some other similar sites, and photos tagged that way at Flickr.
As tags accumulate, it becomes possible to cluster tags in interesting ways. For example, Flickr provides clusters of photos tagged "capri," some of which are photos of the Italian island and some of which are of the Ford Capri car. Flickr figures out which photos go in which by doing a statistical analysis of its tags. Flickr notices, for example, that many photos tagged "capri" are also tagged "island," "italy," "vacation," or beach, while other photos tagged "capri" are also tagged "car," "ford," "automobile" and "vehicle." This provides enough information for Flickr to sort the photos with surprising accuracy.