MARC
From Just Solve the File Format Problem
MARC is a standard for formatting records for cataloging and bibliographic records, used in the library field. It governs the overall structure of the records, with content details being governed by other standards such as the Anglo-American Cataloguing Rules. MARC has been around since the 1960s, and is widely regarded as archaic and inconvenient, but it is difficult to dislodge due to the huge amount of legacy data in this format.
Several versions of MARC exist; MARC 21 is a commonly used one at present.
MARC records are traditionally stored in binary ISO 2709 files, but there is also an XML schema available for it (MARCXML). MADS is an XML schema for describing metadata authority information.
