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)
C. Hoede, and L. Zhang. Proceedings of the 9th International Conference on Conceptual Structures (ICCS 2001), volume 2120 of Lecture Notes in Computer Science, page 15-28. Springer, (2001)
S. Borgo, L. Bozzato, T. Mossakowski, and A. Zimmermann (Eds.) volume 2518 of CEUR Workshop Proceedings, (2019)part of the JOWO 2019 The Joint Ontology Workshops Proceedings of the Joint Ontology Workshops 2019 Episode V: The Styrian Autumn of Ontology Graz, Austria, September 23-25, 2019.