Cataloging deals with the systematic organization of information, data, or materials. It is done by the creation of a full record of bibliographic information about an information object, cross-referenced to other records and files, and includes the process of identifying and documenting these objects in detail.
Cataloging is usually split into descriptive cataloging and subject cataloging. The former attempts to describe what an information object is (a book? how many pages? Size? a DVD? How many minutes of playing time? etc.) according to a set of descriptive cataloging rules such as AACR2, RAK, or Dublin Core, while the latter attempts to describe what the information object is about, usually by assigning subject headings from a controlled vocabulary thesaurus (Library of Congress Subject Headings, Medical Subject Headings (MeSH), Sears List of Subject Headings, etc.) and/or by assigning a classification number from a particular classification scheme, such as Dewey Decimal Classification or Library of Congress Classification.
Catalogers are the people who enter all the information necessary about library materials in the online catalog. Most cataloging is done cooperatively, which is to say, one original cataloger catalogs a particular item once (a role often fulfilled by catalogers at the Library of Congress), and the resultant bibliographic record is then shared through worldwide databases like OCLC and imported into local databases by copy catalogers, catalog librarians, or technical services librarians, usually with little or no modification. Thanks to the ability to share bibliographic information worldwide in real-time, this has greatly reduced the need for original cataloging at most academic libraries, and nearly completely for most public libraries, many of whom accept pre-cataloged, shelf-ready books directly from vendors. In libraries with small technical services staffs, it is common to outsource increasingly more, if indeed not all of the cataloging work.