The web is a chaotic place. Search engines have been extremely popular due to their ability to quickly present the user with relevant results using the keyword occurrence method:the more often the search term appears in the web document the more relevant the web document is to the search (an interesting counter example here) , as well as other methods to attribute authority and rank websites.
An alternative philosophy has merged along with the booming of the so called user generated content. Social bookmarking services allow users to both label their bookmarks with a few words called ‘tags’ and share them with other users. This lead to a new method of searching: the more people tag a website as relevant to something the more it is.
It also gave the possibility to attribute multiple tags to one website giving this way a short summary of its content as viewed by the people having visited and bookmarked it in contrast to what publishers advertise it to be within the site’s metadata.
Although neither this approach of tagging is thought to be a flawless one since people may tag in peculiar ways (What could the tag "ghjkl" mean anyway?) many support as useful this distributed classification system generated by users themselves also called by some a folksonomy.