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)
S. Bleul, T. Weise, and K. Geihs. Special Edition Editorial: Engineering Design and Composition of Service-Oriented Applications, Computer Systems Science & Engineering, (June 2006)
S. Bleul, T. Weise, and K. Geihs. (2006)Accepted for publication in the special issue on Engineering Design and Composition of Service-Oriented Applications.
H. Mouratidis, P. Giorgini, and G. Manson. Proceedings of the KES 2003 Invited Session Ontology and Multiagent Systems Desing (OMASD'03), University of Oxford, United Kingdom, (September 2003)
T. Gruber, and G. Olsen. Proceedings of the 4th International Conference on Principles of Knowledge Representation and Reasoning (KR'94). Bonn, Germany, May 24-27, 1994., page 258-269. Morgan Kaufmann, (1994)
T. Gruber, and G. Olsen. Principles of Knowledge Representation and Reasoning: Proceedings of the 4th International Conference (KR '94): Bonn, Germany: 1994, May, 24 - 27, page 258-269. Morgan Kaufmann Publishers, (1994)
J. Gómez-Romero, F. Bobillo, and M. Delgado. Proceedings of the 6th International Semantic Web Conference and 2nd Asian Semantic Web Conference (ISWC/ASWC2007), Busan, South Korea, volume 4825 of LNCS, page 71--84. Berlin, Heidelberg, Springer Verlag, (November 2007)