Tagging is a relatively new but increasingly popular way for Internet users to organize things by using personal keywords. Tags can be used to label all kinds of items, including books, Web pages and pictures. Already, some are calling tags "the Internet's Dewey Decimal System."
For a book like Gone With the Wind, you might assign tags like "Civil War," "fiction," "epic," and "romance." It all depends on what the book means to you.
Users create tags for their own purposes, but they can be used by anyone. With enough people participating, tags can become an effortless, super-accurate recommendations system among like-minded people.
The site that popularized tagging was #www.Flickr.com#, a social site where users store, organize and share their digital photos. Instead of using a single category for organizing pictures--like a folder labeled "2005 Vacation"--members use one- or two-word tags like waterfall, solar eclipse, Houston, Joe or 2005. This way, photos can be grouped and discovered in multiple ways.
Tags are a form of metadata, which means, literally, "data about data." Tagging creates a folksonomy, a bottom-up method of categorization. Taxonomies are governed by experts like librarians and botanists who want to show hierarchical relationships. Folksonomies are built by amateurs but can be more helpful for users.
Folksonomies are gaining steam, aided by the easy exchange of ideas online. Often taxonomies aren't specific, flexible or current enough. Increasingly, people use tags to tap collective wisdom.