A library employee, typically a librarian, who manages the bibliographic records that describe the items in the library's collection.
A cataloger may often get her bibliographic records from a book vendor that provides this value-added service for a small additional fee. When this is not available, a good cataloger will copy an existing record from other systems: the Library of Congress, local libraries, and so on. If no records exist, the cataloger checks for Cataloging in Publication data, and failing that, she creates a new bibliographic record to describe it.
Catalogers must master the arcane symbols of the Dewey Decimal System, the MARC format, and other technical details. For this reason, they often have quite a bit in common with programmers.