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)