What is a tag?
In online computer systems terminology a tag is a keyword or term assigned to a piece of information (such as an internet bookmark, digital image, or computer file). A tag 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. (wikipedia.org)
What is Taxonomy?
Taxonomy is the practice and science of classification. The word comes from two Greek words that mean "the science of order." Taxonomies are frequently hierarchical in structure, meaning they often go from a general term to a more specific term. Taxonomies are structured and defined by some type of authority. For example, the Dewey Decimal System is a taxonomy and is maintained by the Online Computer Library Center (OCLC) of Dublin, Ohio which works in collaboration with the Library of Congress to revise it every seven years. This type of classification is controlled. Thus all books in libraries that use the Dewey system are classified in the same way. From: Joris Poelmans JOPX on SharePoint 2007 (MOSS and WSS V3 )
What is a Folksonomy?
A Folksonomy is a user generated taxonomy used to categorize and retrieve web content such as Web pages, photographs and Web links, using open ended labels called tags. Typically, folksonomies are Internet-based, but their use may occur in other contexts. The folksonomic tagging is intended to make a body of information increasingly easy to search, discover, and navigate over time. A well-developed folksonomy is ideally accessible as a shared vocabulary that is both originated by, and familiar to, its primary users. However, no one authority really controls the folksonomy. Two widely cited examples of websites using folksonomic tagging are Flickr and del.icio.us. From: Joris Poelmans JOPX on SharePoint 2007 (MOSS and WSS V3 )