Folksonomy is a neologism formed with the words folk and taxonomy. It is the classification of online content based on user-generated tags. This classification can be the work of a single individual, but more often it refers to the cooperation among many individuals of a particular online service or community. One example of a folksonomy is the classification of the pictures on Flickr.com.
In social tagging, users of a service provide the tags, or labels, that describe the content (of photos, Web links, art), thus creating a user-generated taxonomy, or folksonomy, as it’s called. (NY Times)