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)