Classification or document categorization, is a problem in library science, information science and computer science. Things may be classified according to their subjects or according to other attributes (such as document type, author, printing year etc.). — Wikipedia
Controlled vocabularies provide a way to organize resources for subsequent retrieval. They are used in subject indexing schemes, subject headings, thesauri, taxonomies and other knowledge organization systems. Controlled vocabulary schemes mandate the use of predefined, authorized terms that have been preselected by the designers of the schemes, in contrast to natural language vocabularies, which have no such restriction. — Wikipedia
Encyclopedia of Knowledge Organization is an online, open access encyclopedia of knowledge organization hosted by International Society for Knowledge Organization and produced by the International Society for Knowledge Organization’s Scientific Advisory Council. It contains entries about concepts, disciplines, organizations, systems, standards, and theories etc. including important domain specific systems. The encyclopedia shall span the field of Knowledge Organization from classical concepts and systems to up-to-date advanced technologies, concepts and issues. — International Society for Knowledge Organization
Basic Register of Thesauri, Ontologies & Classifications (BARTOC) is a database of Knowledge Organization Systems and KOS related registries. The main goal of BARTOC is to list as many Knowledge Organization Systems as possible at one place in order to achieve greater visibility, highlight their features, make them searchable and comparable, and foster knowledge sharing. BARTOC includes any kind of KOS from any subject area, in any language, any publication format, and any form of accessibility. BARTOC’s search interface is available in 20 European languages and provides two search options: Basic Search by keywords, and Advanced Search by taxonomy terms. A circle of editors has gathered around BARTOC from all across Europe and BARTOC has been approved by the International Society for Knowledge Organization (ISKO). — BARTOC.org
Here are links to pages about closely related subjects.