Folksonomy is a neologism for a practice of collaborative categorization using simple tags. This feature has begun appearing in a variety of social software. At present, the best examples of online folksonomies are social bookmarking sites like del.icio.us, a bookmark sharing site, Flickr, for photo sharing, and 43 Things, for goal sharing. Gmail's labeling system is somewhat similar to the use of tags, but it is not a folksonomy because users cannot share their categorizations. Folksonomy is not directly related to the concept of faceted classification from library science.
Folksonomy is currently understood somewhat narrowly as "tagging". Social sciences and anthropology have long studied "folk classifications" - how average people (non-experts) classify the world around them.