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)
A. Hotho, R. Jäschke, C. Schmitz, and G. Stumme. Proceedings of the First Conceptual Structures Tool Interoperability Workshop at the 14th International Conference on Conceptual Structures, page 87-102. Aalborg, Aalborg Universitetsforlag, (2006)
L. Specia, and E. Motta. Proceedings of the European Semantic Web Conference (ESWC2007), volume 4519 of LNCS, page 624-639. Berlin Heidelberg, Germany, Springer-Verlag, (July 2007)
D. Laniado, D. Eynard, and M. Colombetti. Semantic Web Application and Perspectives - Fourth Italian Semantic Web Workshop, page 192--201. (December 2007)