Thesauri and ontologies are tools for content description and topic-based information retrieval of both printed and electronic materials. They assist data entry and retrieval specialists  to use a common language in order to find the required information more easily.

Thesauri are used above all in the databases of libraries, archives and museums. They may be general thesauri or thesauri for special fields.

Ontologies are tools for describing information resources and for information retrieval, especially in semantic web environments.

Ontology refers to a hierarchy of concepts that describes concepts and the relations between them in some topic area in a machine-readable format, and it identifies the concepts with identifiers instead of terms.

In an ontology – unlike a thesaurus  – the relations between concepts have been defined comprehensively and hierarchically for the whole set of concepts, and the highest level of the ontology hierarchy collects together all the concepts in the ontology system. The concept hierarchy comprises generic (species–subspecies) and partitive (part–whole) relations between concepts.

In ontologies, generic relations are usually talked about as umbrella and subordinate relations. Ontologies may also have associative relationships, which convey that the concepts are related, but not hierarchically.

The National Library of Finland maintains and develops the following thesauri and ontologies:

The National Library of Finland has technical and/or coordination responsibility for

The content of the following is not maintained any more (deprecated vocabularies):

  • General Finnish Thesaurus (YSA)
  • General Finnish Thesaurus in Swedish (Allärs)
  • Finnish Music Thesaurus (MUSA)
  • Special thesaurus for music in Swedish (CILLA)
  • Ontology for Music (MUSO)
  • Finnish Corporate Names

Thesauri and ontologies are available via the ontology service maintained by the National Library of Finland.