What’s the difference between a controlled vocabulary, a taxonomy, a thesaurus, and an ontology? We’ve found people using some of these terms interchangeably, and it’s difficult to find reliable official definitions for each.
We’ve come up with some unofficial working definitions that have served us well when discussing different kinds of controlled vocabularies and their associated metadata. Our new white paper Controlled vocabularies, taxonomies, and thesauruses (and ontologies) describes each of these and which TopQuadrant products give customers the control they need over the kinds of vocabularies and metadata that they’re working with.
We hope that this short white paper gives you a better idea of why people build, use, and store controlled vocabularies and the advantages that standards bring to this work.

Roget's Thesaurus