According to the Library of Congress Network Development and MARC Standards Office the MARC (MAchine-Readable Cataloging) formats "are standards for the representation and communication of bibliographic and related information in machine-readable form." You'll be experiencing MARC formats as a collection of numbers and letters used to code and manipulate bibliographic, authority, and holdings information in both the NOTIS and OCLC systems.
Both NOTIS and OCLC use their own versions of the MARC format. They are very similar to the "standard" MARC format you'll be learning about in this page. There are tables and links to documentation explaining the differences in these Web pages.
The MARC record explained
For quick reference here are links to MARC documentation:
USMARC Concise format for Bibliographic Data
USMARC Concise format for Authority Data
Book Cataloger's Cheat-Sheet
Score Cataloger's Cheat-Sheet