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. Staab, J. Angele, S. Decker, A. Hotho, A. Maedche, H. Schnurr, R. Studer, and Y. Sure. AAAI 2000/IAAI 2000 - Proceedings of the 17th National Conference on Artificial Intelligence and 12th Innovative Applications of Artificial Intelligence Conference, Austin/TX, USA, July 30-August 3, 2000, AAAI Press/MIT Press, (2000)
S. Staab, J. Angele, S. Decker, A. Hotho, A. Maedche, H. Schnurr, R. Studer, and Y. Sure. AAAI 2000/IAAI 2000 - Proceedings of the 17th National Conference on Artificial Intelligence and 12th Innovative Applications of Artificial Intelligence Conference, Austin/TX, USA, July 30-August 3, 2000, AAAI Press/MIT Press, (2000)
S. Staab, J. Angele, S. Decker, A. Hotho, A. Maedche, H. Schnurr, R. Studer, and Y. Sure. AAAI 2000/IAAI 2000 - Proceedings of the 17th National Conference on Artificial Intelligence and 12th Innovative Applications of Artificial Intelligence Conference, Austin/TX, USA, July 30-August 3, 2000, AAAI Press/MIT Press, (2000)
S. Staab, J. Angele, S. Decker, A. Hotho, A. Maedche, H. Schnurr, R. Studer, and Y. Sure. AAAI 2000/IAAI 2000 - Proceedings of the 17th National Conference on Artificial Intelligence and 12th Innovative Applications of Artificial Intelligence Conference, Austin/TX, USA, July 30-August 3, 2000, AAAI Press/MIT Press, (2000)
S. Staab, J. Angele, S. Decker, A. Hotho, A. Maedche, H. Schnurr, R. Studer, and Y. Sure. AAAI 2000/IAAI 2000 - Proceedings of the 17th National Conference on Artificial Intelligence and 12th Innovative Applications of Artificial Intelligence Conference, Austin/TX, USA, July 30-August 3, 2000, AAAI Press/MIT Press, (2000)
C. Kiefer, A. Bernstein, and A. Locher. Proceedings of the 5th European Semantic Web Conference (ESWC 2008), 5021, page 478-492. Tenerife, Spain, Springer Verlag, (2008)