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)
G. Qi, and F. Yang. Proceedings of the 2nd International Conference on Web Reasoning and Rule Systems (RR 2008), volume 5341 of Lecture Notes in Computer Science, page 74--88. Karlsruhe, Germany, Springer-Verlag, (October 2008)