The MAchine Readable Cataloging format was designed by the Library of
Congress in the late 1960s in order to allow libraries to convert their card
catalogs into a digital format. The advantages of having computerized card
catalogs were soon realized, and now MARC is being used by all sorts of
libraries around the world to provide computerized access to their collections.
MARC data in transmission format is optimized for processing by computers, so 
it's not very readable for the normal human. For more about the MARC format,
visit the Library of Congress at http://www.loc.gov/marc/