Online public access catalog in the context of "Card catalog"

A library catalog (or library catalogue in British English) is a register of all bibliographic items found in a library or group of libraries, such as a network of libraries at several locations. A catalog for a group of libraries is also called a union catalog. A bibliographic item can be any information entity (e.g., books, computer files, graphics, realia, cartographic materials, etc.) that is considered library material (e.g., a single novel in an anthology), or a group of library materials (e.g., a trilogy), or linked from the catalog (e.g., a webpage) as far as it is relevant to the catalog and to the users (patrons) of the library.

The earliest library catalogs were lists, handwritten or enscribed on clay tablets and later scrolls of parchment or paper. As codices (books with pages) replaced scrolls, so too did library catalogs become like handwritten ledgers and, in some cases, printed books. During the late 18th century through mid-19th century, cataloguing on paper slips or cards gradually replaced ledgers and books as the main medium for library catalogs, and in the 20th it was long ubiquitous. The card catalog was a familiar sight to library users for generations. Computerized cataloguing developed gradually from the mid-20th, and by the late 20th and early 21st, it had mostly replaced card catalogs. The advent of the web brought about ubiquitous use of online public access catalogs (OPACs). Some people still informally refer to the online catalog as a "card catalog".

OCLC, Inc. is an American nonprofit cooperative organization "that provides shared technology services, original research, and community programs for its membership and the library community at large". It was founded in 1967 as the Ohio College Library Center, then became the Online Computer Library Center as it expanded. In 2017, the name was formally changed to OCLC, Inc. OCLC and thousands of its member libraries cooperatively produce and maintain WorldCat, the largest online public access catalog in the world. OCLC is funded mainly by the fees that libraries pay (around $217.8 million annually in total as of 2021) for the many different services it offers. OCLC also maintains the Dewey Decimal Classification system.