I just had a reference question via Aardvark: “What is the difference between a folksonomy and an ontology?” (Obviously, this is someone in the midst of a library science program.) So, I told the person:
A folksonomy is a form of distributed, non-hierarchical, uncontrolled classification system. These terms each have precise technical meanings. A folksonomy is distributed, meaning that the task of providing classificatory terms is spread among two or more agents. A folksonomy is non-hierarchical, meaning there are no parent-child relationships in the classification structure. A folksonomy is uncontrolled, meaning the classificatory terms are neither predefined nor exclusive (you can have any level of specificity and use any number of synonyms).
Ontology, on the other hand, is the branch of metaphysics that examines the what sorts of entities exist, as well as examines the nature of being. Ontology is that area of philosophy where we discuss the problem of universals, the existence of numbers, the existence of gods, etc.
And the ‘difference’ between them? Well, that’s an irrelevant question. The real question should be, “Why has ‘ontology’ been co-opted?”