While tags in collaborative tagging systems serve primarily an indexing purpose, facilitating search and navigation of resources, the use of the same tags by more than one individual can yield a collective classification schema.

— Lucia Specia; Enrico Motta, Integrating Folksonomies with the Semantic Web, The Semantic Web: Research and Applications. ESWC 2007. Lecture Notes in Computer Science, Vol. 4519, 2007, p. 624–639, (via)