Cloud of knowledge
"Social tagging" is a form of classification in which users categorize objects – photos, websites, scientific papers – by freely choosing a set of "tags". For example, you might tag a photo with the words "family", "holiday" and "Spain". It is then possible to search using the tags that other users have added. This is in contrast to a strict "top-down" classification system like the Dewey decimal system for books. A "tag cloud" represents the most popular tags by displaying the words in different sizes depending on how often they are used. The tag cloud shown here is taken from the Connotea site, which allows users to tag scientific papers.