Knowledge organization (KO) refers to activities like document description, indexing and classification performed in libraries and databases to organize documents and concepts. Knowledge organization systems (KOS) include classification schemes, subject headings, thesauri and other systems used to organize information. KOS impose structures on collections and can be used in digital libraries to provide overviews and support retrieval, though different KOS may characterize entities differently. Common types of KOS include term lists, classifications and categories, and relationship lists.