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. Sahebi, F. Oroumchian, und R. Khosravi. WI-IAT '08: Proceedings of the 2008 IEEE/WIC/ACM International Conference on Web Intelligence and Intelligent Agent Technology, Seite 82--85. Washington, DC, USA, IEEE Computer Society, (2008)
A. Isaac, L. der Meij, S. Schlobach, und S. Wang. Proceedings of the 6th International Semantic Web Conference and 2nd Asian Semantic Web Conference (ISWC/ASWC2007), Busan, South Korea, Volume 4825 von LNCS, Seite 252--266. Berlin, Heidelberg, Springer Verlag, (November 2007)