Gruber said he saw four different ways of adding meta-tags to material, ranging from the loosest to the most strict:
1.Folksonomy (informal, user-defined)
2.Controlled vocabularies (the user must deploy a set of defined terms)
3.Taxonomy (Pre-defined terms in which specific terms are subsets of more general terms)
4.Ontology (A rich set of relationships is mapped out among all the terms)
J. Rilling, Y. Zhang, W. Meng, R. Witte, V. Haarslev, and P. Charland. Models in Software Engineering: Workshops and Symposia at MoDELS 2006, Genoa, Italy, October 1-6, 2006, Reports and Revised Selected Papers, volume 4364 of LNCS, page 56--65. Springer Berlin/Heidelberg, (2007)
G. Guizzardi, and G. Wagner. CAiSE Workshops, page 129-143. Faculty of Computer Science and Information Technology, Riga Technical University, Riga, Latvia, (2004)