Controlled vocabularies
Thesauri and classifications are documentation languages that are applied for content description of objects (e.g. research data). Apart from a standardized metadata schema, they significantly enhance metadata. A classification attends the placement in (usually hierarchical) classes that are characterized by certain traits. A thesaurus (pl. thesauri) is a natural linguistic, ordered collection of terms and their relationship to each other. The use of controlled keywords from a thesaurus and the classification of content to a specific field facilitates the findability of research data. For many departments, there are already own specialized classifications and thesauri.
The Basel Register of Thesauri, Ontologies & Classifications (http://www.bartoc.org/) offers a search and has many recognized thesauri and classifications listed.
Below you find some examples of classifications and thesauri.
Classifications
General
Dewey Decimal Classification (DDC), http://web.archive.org/web/20130130074038/http://www.oclc.org/dewey/resources/summaries/default.htm
Library of Congress Classification (LCC), http://www.loc.gov/catdir/cpso/lcco/
By discipline
Environment Classification, https://sns.uba.de/umthes/de/collections/UK.html
Classification for Social Sciences, http://www.gesis.org/unser-angebot/recherchieren/tools-zur-recherche/klassifikation-sozialwissenschaften/
Journal of Economic Literature Classification System (JEL), http://www.aeaweb.org/jel/guide/jel.php
Physics Subject Headings (PhySH), https://physh.aps.org/
Mathematics Subject Classification (MSC), https://www.zbmath.org/classification/
Thesauri
Agricultural Sciences: AGROVOC Multilingual agricultural thesaurus, http://aims.fao.org/vest-registry/vocabularies/agrovoc-multilingual-agricultural-thesaurus
Humanities: A Thesaurus of Old English, http://oldenglishthesaurus.arts.gla.ac.uk/
Information Sciences: INFODATA Thesaurus, http://www.infodata-edepot.de/thesaurus/T_SM.HTM
Art and Architecture: Art and Architecture Thesaurus (AAT), http://www.aat-deutsch.de/
Thesaurus for Geographic Names (TGN),
http://www.getty.edu/research/tools/vocabularies/tgn/index.html
Life Sciences: Environmental Thesaurus (UMTHES), https://sns.uba.de/umthes/de.html
Medicine and Biosciences: Thesaurus Medical Subject Headings (MeSH), https://www.nlm.nih.gov/mesh/
Psychology: Thesaurus Psychology (PSYNDEX), http://www.zpid.de/index.php?wahl=products&uwahl=printed&uuwahl=psyndexterms
Social Sciences: Thesaurus Social Sciences (TheSoz), http://www.gesis.org/en/services/research/tools/social-science-thesaurus/
Economics: Standard Thesaurus Economics (STW), http://zbw.eu/stw/version/latest/about