Social tagging, which is also known as collaborative tagging, social classification, and social indexing, allows ordinary users to assign keywords, or tags, to items.
G. Forman, und E. Kirshenbaum. CIKM '08: Proceeding of the 17th ACM conference on Information and knowledge management, Seite 1221--1230. New York, NY, USA, ACM, (2008)