: 142 : 24 Top-down taxonomies are created by an authorized group of designers (sometimes in the form of a controlled vocabulary), whereas bottom-up taxonomies (called folksonomies) are created by all users. Tagging systems have sometimes been classified into two kinds: top-down and bottom-up. On websites that aggregate the tags of all users, an individual user's tags can be useful both to them and to the larger community of the website's users.

Websites that include tags often display collections of tags as tag clouds, as do some desktop applications. These sites allow users to create and manage labels (or 'tags') that categorize content using simple keywords. Tagging gained popularity due to the growth of social bookmarking, image sharing, and social networking websites. Computer based search algorithms made the use of such keywords a rapid way of exploring records. People were using textual keywords to classify information and objects long before computers. An analogous example of tags in the physical world is museum object tagging. Tags may take the form of words, images, or other identifying marks. People use tags to aid classification, mark ownership, note boundaries, and indicate online identity.