In online computer systems terminology, a tag is a non-hierarchical keyword or term assigned to a piece of information (such as an internet bookmark, digital image, or computer file). This kind of metadata helps describe an item and allows it to be found again by browsing or searching. Tags are chosen informally and personally by the item's creator or by its viewer, depending on the system.

Tagging was popularized by websites associated with Web 2.0 and is an important feature of many Web 2.0 services. It is now also part of some desktop software. Labeling and tagging are carried out to perform functions such as aiding in classification, marking ownership, noting boundaries, and indicating identity [disambiguation needed].

They may take the form of words, images, or other identifying marks. Analogous examples of tags in the physical world are graffiti and museum object tagging. In the organisation 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 free-form 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.

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