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OWL: Web Ontology Language
The Web Ontology Language (OWL) is a languge devised for applications that need to process the data rather than just displaying it to humans. Web ontologies describe the relationships between entities, usually referred to as "kinds", and how the different kinds of entities are related.
OWL Web Ontology Language Overview
The W3C explains OWL and how it works.
Wikipedia: Web Ontology Language
Wikipedia explains the Web Ontology Language and provides some background.