Ontologies are semantic models in Semantic Web

  • an ontology is some knowledge about concepts of some domain and the relations between them
  • or, an ontology is a formal description that provides a user some (shared) understanding about a domain

Ontologies for Semantic Web:

  • should be understandable by machines
  • should allow reasoning
  • RDFS + OWL for describing ontologies on top of RDF graphs
  • Tbox in Descriptive Logic as the formal foundation for inferencing
  • in OWL there's a special property: SOME_URI a owl:Ontology

Useful for


  • upper ontologies - general things in abstract manner
  • domain ontologies
  • lightweight ontologies for web

Popular Ontologies


FOAF - friend of a friend (uses RDFS-Plus)

  • a format for describing people and their relationships
  • also, about organizations


SKOS (uses RDFS-Plus)

  • Simple Knowledge Organization System
  • e.g. controlled vocabularies, taxonomies, thesauri - defines relationships between terms
  • in a distributed and linkable way


  • GR: Good Relations - for business to make descriptions of their offers (uses OWL)
  • QUDT - Quantity/Units/Dimensions/Types - for aligning data coming from multiple source (uses OWL)

See Also