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. Gal, и P. Shvaiko. Advances in Web Semantics I: Ontologies, Web Services and Applied Semantic Web, том 4891 из Lecture Notes in Computer Science, Springer, Berlin, (2009)
C. Kiefer, A. Bernstein, и A. Locher. Proceedings of the 5th European Semantic Web Conference (ESWC 2008), 5021, стр. 478-492. Tenerife, Spain, Springer Verlag, (2008)
F. Shi, J. Li, J. Tang, G. Xie, и H. Li. The Semantic Web -- ISWC 2009: 8th International Semantic Web Conference, Chantilly, VA, USA, том 5823 из Lecture Notes in Computer Science, Springer, Berlin, (2009)
C. Schmitz, S. Staab, R. Studer, G. Stummen, и J. Tane. Proc. of E-Learning 2002 World Conference on E-Learning in Corporate, Government, Healthcare and Higher Education on (E-Learning 2002), AACE, стр. 909-915. Norfolk, (2002)Awarded paper.
D. Jakobsen, P. Øhrstrøm, и H. Schärfe. Proceedings of the 19th International Conference on Conceptual Structures (ICCS 2011), том 6828 из Lecture Notes in Computer Science, стр. 118-130. Springer, (2011)