Dewey Decimal Classification
The Dewey Decimal Classification (DDC) system is the world's most widely used library classification system. It was created by librarian Melville Dewey in the 1872s and has been owned by OCLC since 1988. "The DDC system has been translated into more than 30 languages and serves library users in more than 200,000 libraries in 135+ countries worldwide, making it the world's most widely used library classification system. More than 60 of these countries use the DDC to organize their national bibliographies." - Quote from the OCLC Website
It is a system of classifying books and other works which divides human knowledge into ten basic categories, with subdivisions indicated by decimal notation. Dewey Decimal call numbers begin with Arabic numerals.
- Sample call number: 996.902 H75c
As of this writing, the DDC is in its 22nd edition, commonly abbreviated DDC22. The complete DDC schedule is published in hardcopy, normally in four complete volumes, but with DDC22, it is also available online through the web-based utility known as WebDewey, which is also owned and licensed by OCLC.
Also, as of January 26, 2015 the DDC is now in its 23rd edition, abbreviated DDC 23 according to the OCLC Website.  The unabridged, four volume set is usually published every seven years. This edition is also available online through WebDewey.