Answer:
Library Domain Model describes main classes and relationships which could be used during analysis phase to better understand domain area for Integrated Library System (ILS), also known as a Library Management System (LMS).
An Online Public Access Catalog (OPAC) is e-Library website which is part of Integrated Library System or Library Management System, and managed by a library or group of libraries.
Patrons of the library can search library catalog online to locate various resources - books, periodicals, audio and visual materials, or other items under control of the library. Patrons may reserve or renew item, provide feedback, and manage their account.
See links below with examples.