From the Library's announcement:
“The Library of Congress is our nation’s monument to knowledge and we need to make sure the doors are open wide for everyone, not just physically but digitally too,” said Librarian of Congress Carla Hayden. “Unlocking the rich data in the Library’s online catalog is a great step forward. I’m excited to see how people will put this information to use.”
The new, free service will operate in parallel with the Library’s fee-based MARC Distribution Service, which is used extensively by large commercial customers and libraries. All records use the MARC (Machine Readable Cataloging Records) format, which is the international standard maintained by the Library of Congress with participation and support of libraries and librarians worldwide for the representation and communication of bibliographic and related information in machine-readable form.
The data covers a wide range of Library items including books, serials, computer files, manuscripts, maps, music and visual materials. The free data sets cover more than 45 years, ranging from 1968, during the early years of MARC, to 2014. Each record provides standardized information about an item, including the title, author, publication date, subject headings, genre, related names, summary and other notes.