Allyson Carlyle, a professor at the Information School at the University of Washington, has a paper called Understanding FRBR as a Conceptual Model: FRBR and the Bibliographic Universe in the October 2006 issue of Library Resources and Technical Services (50: 4). (The link is to a web version of the paper; the illustrations are at the bottom.) I recommend it.
The blurb about the author says the paper is based on a 2004 talk (which was mentioned here in June 2005). Carlyle says, “It is the outcome of six years of teaching the FRBR model, answering student questions about it, and explaining it to inquisitive faculty who return from conferences at which it is mentioned.”
[Abstract:] Functional Requirements for Bibliographic Records (FRBR) presents a complex conceptual model. Because of this, it is not easy for everyone to understand. The purpose of this paper is to make some of the more difficult aspects of the FRBR model, in particular the Group 1 entities work, expression, manifestation, and item, easier to understand by placing FRBR in the context of what it is: a conceptual entity-relationship model. To this end, a definition of the term “model” is presented, a variety of types and functions of models are introduced, conceptual models are discussed in detail, modeling an abstraction is explained, and different ways of interpreting FRBR are suggested. Various models used in the history of cataloging are introduced to place FRBR in the context of the historical development of document models.