An ontology, like a thesaurus, is a kind of taxonomy with structure and specific types of relationships between terms. In an ontology the types of relationships are greater in number and more specific in their function. Relationships could include, for example, located in to relate an organization to a place, produces/is produced by to relate a company and its product, and employs/employed by to relate a company and a person. Information, that in a simple controlled vocabulary or taxonomy is conveyed through indexing, is embedded into the ontology itself.

Ontological relationships are used in more complex information systems, such as the Semantic Web.