Creating a catalog record for an item without the aid of another library's record (called copy cataloging). Original cataloging is most often needed for unique items, such as materials pertaining to local history, archival materials, rare books, manuscripts, maps, some sound recordings, etc.
Original cataloging takes a good deal of training and apprenticeship. It requires proficiency both in descriptive cataloging and subject cataloging, and a strong familiarity with common cataloging tools such as AACR2, Library of Congress Rule Interpretations (LCRIs), the five volume Library of Congress Subject Headings (LCSH) or other common subject thesaurus (Sears, etc.), MARC21 standards, and at least one classification scheme such as LCC or DDC or UDC.
Catalogers who are familiar with advanced subjects, managing electronic resources, emergent metadata schemes and/or fluent in multiple languages can be in high demand.