The standards for the representation and exchange of bibliographic, authority, and holdings data in machine-readable form in the United States are the three USMARC communications formats for Bibliographic Data, Authority Data, and Holdings Data.
A USMARC record, hereafter referred to simply as MARC, is composed of three elements: record structure, content designation, and data content. The record structure is an implementation of the American National Standard for Bibliographic Information Interchange (ANSI Z39.2). Content designation (the codes and conventions established to identify data elements within a record and to support manipulation of that data) is defined by each of the MARC formats. The actual data content of a MARC record is defined by standards outside the bibliographic formats, including cataloging codes, classification schedules, and controlled subject heading lists.
At the individual record level, the main components of a MARC record are a leader (24 characters), record directory (12 characters for each variable field), variable control fields, and variable data fields. Each variable field ends with a field terminator character and the last field ends with a field terminator character followed by a record terminator character.