MARC Records
MARC stands for MAchine-Readable Cataloging and serves as the language of library catalog records.
The anatomy of MARC records
MARC records have three main elements: a field that contains information about the item (such as "William Shakespeare"), a tag that tells the computer what the field represents (such as "100" for author), and a set of indicators that provide further information about the field. For example, tag 005 in the image to the right tells the computer that the following number string (20160615082311.0, or 2016/06/15 08:23:11) is the last time this item was last checked out or moved. Koha helpfully labels this field for us so we don't have to memorize the tags.
Some fields in the records also contain subfields that contain additional information. Like the fields and their tags above, each subfield has a subfield code that tells the computer what kind of information follows. The subfields are indicated by a delimiter, usually a $, which is not shown on the Koha page, because the subfields are divided by line breaks. Tag 040 indicates that the following fields contain the catalogers of the item. Subfield code a indicates that the subfield contains the original cataloging agency. Subfield code b indicates the cataloging language, etc. You can see how helpful Koha is in providing labels. See Understanding MARC Bibliographic for more information.