A list of classes arranged according to a set of pre-established principles for the purpose of organizing items in a collection, or entries in an index, bibliography, or catalog, into groups based on their similarities and differences, to facilitate access and retrieval. In the United States, most library collections are classified by subject. Classification systems can be enumerative or hierarchical, broad or close. In the United States, most public libraries use Dewey Decimal Classification, but academic and research libraries prefer Library of Congress Classification. See also: Classification Society of North America, Colon Classification, and notation.


From ODLIS, Online Dictionary for Libary and Information Science by Joan M. Reitz
http://lu.com/odlis/about.cfm