History and context of tags

Labeling and tagging are carried out to perform functions such as aiding in classification, marking ownership, noting boundaries, and indicating online identity. They may take the form of words, images, or other identifying marks. An analogous example of tags in the physical world is museum object tagging. In the organization of information and objects, the use of textual keywords as part of identification and classification long predates computers. However, computer based searching made the use of keywords a rapid way of exploring records.

Online and Internet databases and early websites deployed them as a way for publishers to help users find content.


In 2003, the social bookmarking website Delicious provided a way for its users to add "tags" to their bookmarks (as a way to help find them later); Delicious also provided browseable aggregated views of the bookmarks of all users featuring a particular tag. Flickr allowed its users to add their own text tags to each of their pictures, constructing flexible and easy metadata that made the pictures highly searchable. The success of Flickr and the influence of Delicious popularized the concept, and other social software websites – such as YouTube, Technorati, and Last.fm – also implemented tagging. "Labels" in Gmail are similar to tags.

Websites that include tags often display collections of tags as tag clouds. A user's tags are useful both to them and to the larger community of the website's users.

Tags may be a "bottom-up" type of classification, compared to hierarchies, which are "top-down". In a traditional hierarchical system (taxonomy), the designer sets out a limited number of terms to use for classification, and there is one correct way to classify each item. In a tagging system, there are an unlimited number of ways to classify an item, and there is no "wrong" choice. Instead of belonging to one category, an item may have several different tags. Some researchers and applications have experimented with combining structured hierarchy and "flat" tagging to aid in information retrieval