An ILS is usually comprised of a relational database, software to act on that database, and two graphical user interfaces (one for patrons, one for staff). Most ILSes separate software functions into discrete programs called modules, which are then integrated into a unified interface. Examples of modules include: acquisitions (ordering, receiving, and invoicing materials), cataloging (classifying and indexing materials), circulation (lending materials to patrons and receiving them back), serials (tracking magazine and newspaper holdings), and the OPAC (public interface for users). Each patron and item has a unique ID in the database that allows the ILS to track its activity.
