There are at least four different meanings to the term work as it is being discussed in library venues.
"Work-ness"First there is the concept that every resource embodies something that could be called a "work" and that this work is a human creation. The idea of the work probably dates back as far as the recognition that humans create things, and that those things have meaning. There is no doubt that there is "work-ness" in all created things, although prior to FRBR there was little attempt to formally define it as an aspect of bibliographic description. It entered into cataloging consciousness in the 20th century: Patrick Wilson saw works as families of resources that grow and branch with each related publication; Richard Smiraglia looked at works as a function of time; and Seymour Lubetzky seems to have been the first to insist on viewing the work as intellectual content separate from the physical piece.
"Work Description"Second, there is the work in the bibliographic description: the RDA cataloging rules define the attributes or data elements that make up the work description, like the names of creators and the subject matter of the resource. Catalogers include these elements in descriptive cataloging even when the work is not defined as a stand-alone entity, as in the case of doing RDA cataloging in a MARC21 record environment. Most of the description of works is not new; creators and subjects have been assigned to cataloged items for a century or more. What is changed is that conceptually these are considered to be elements of the work that is inherent in the resource that is being cataloged but not limited to the item in hand.
It is this work description that is addressed in FRBR. The FRBR document of 1998 describes the scope of its entities to be solely bibliographic, specifically excluding authority data:
"The present study does not analyse those additional data associated with persons, corporate bodies, works, and subjects that are typically recorded only in authority records."Notably, FRBR is silent on the question of whether the work description is unique within the catalog, which would be implied by the creation of a work authority "record".
"Work Decision"Next there is the work decision: this is the situation when a data creator determines whether the work to be described needs a unique and unifying entry within the stated cataloging environment to bring together exemplars of the same work that may be described differently. If so, the cataloger defines the authoritative identity for the work and provides information that distinguishes that work from all other works, and that brings together all of the variations of that work. The headings ("uniform titles") that are created also serve to disambiguate expressions of the same work by adding dates, languages, and other elements of the expression. To back all of this up, the cataloger gives evidence of his/her decision, primarily what sources were consulted that support the decision.
In today's catalog, a full work decision, resulting in a work authority record, is done for only a small number of works, with the exception of musical works where such titles are created for nearly all. The need to make the work decision may vary from catalog to catalog and can depend on whether the library holds multiple expressions of the work or other works that may need clarification in the catalog. Note that there is nothing in FRBR that would indicate that every work must have a unique description, just that works should be described. However, some have assumed that the FRBR work is always a representation of a unique creation. I don't find that expressed in FRBR nor the FRBR-LRM.
"Work Entity"Finally there is the work entity: this is a data structure that encapsulates the description of the work. This data structure could be realized in any number of different encodings, such as ISO 2709 (the underlying record structure for MARC21), RDF, XML, or JSON. The latter two can also accommodate linked data in the form of RDFXML or JSON-LD.
Here we have a complication in our current environment because the main encodings of bibliographic data, MARC21 and BIBFRAME, both differ from the work concept presented in FRBR and in the RDA cataloging rules, which follow FRBR fairly faithfully. With a few exceptions, MARC21 does not distinguish work elements from expression or manifestation elements. Encoding RDA-defined data in the MARC21 "unit record" can be seen as proof of the conceptual nature of the work (and expression and manifestation) as defined in FRBR.
BIBFRAME, the proposed replacement for MARC21, has re-imagined the bibliographic work entity, departing from the entity breakdown in FRBR by defining a BIBFRAME work entity that tends to combine elements from FRBR's work and expression. However, where FRBR claims a neat divison between the entities, with no overlapping descriptive elements, BIBFRAME 2.0 is being designed as a general bibliographic model, not an implementation of FRBR. (Whether or not BIBFRAME achieves this goal is another question.)
The diagrams in the 1998 FRBR report imply that there would be a work entity structure. However, the report also states unequivocally that it is not defining a data format.(**) In keeping with 1990's library technology, FRBR anticipates that each entity may have an identifier, but the identifier is a descriptive element (think: ISBN), not an anchor for all of the data elements of the entity (think: IRI).
As we see with the implementation of RDA cataloging in the MARC21 environment, describing a work conceptually does not require the use of a separate work "record." Whether work decisions are required for every cataloged manifestation is a cataloging decision; whether work entities are required for every work is a data design decision. That design decision should be based on the services that the system is expected to render. The "entity" decision may or may not require any action on the part of the cataloger depending on the interface in which cataloging takes place. Just as today's systems do not store the MARC21 data as it appears on the cataloger's screen, future systems will have internal data storage formats that will surely differ from the view in the various user interfaces.
"The Upshot"We can assume that every human-created resource has an aspect of work-ness, but this doesn't always translate well to bibliographic description nor to a work entity in bibliographic data. Past practice in relation to works differs significantly from, say, the practice in relation to agents (persons, corporate bodies) for whom one presumes that the name authority control decision is always part of the cataloging workflow. Instead, work "names" have been inconsistently developed (with exceptions, such as in music materials). It is unclear if, in the future, every work description will be assumed to have undergone a "work name authority" analysis, but even more unreliable is any assumption that can be made about whether an existing bibliographic description without a uniform title has had its "work-ness" fully examined.
This latter concern is especially evident in the transformations of current MARC21 cataloging into either RDA, BIBFRAME, or schema.org. From what I have observed, the transformations do not preserve the difference between a manifestation title that does not have a formal uniform title to represent the work, and those titles that are currently coded in MARC21 fields 130, 240, or the $t of an author/title field. Instead, where a coded uniform title is not available in the MARC21 record, the manifestation title is copied to the work title element. This means that the fact that a cataloger has carefully crafted a work title for the resource is lost. Even though we may agree that the creation of work titles has been inconsistent at best, copying transcribed titles to the work title entity wherever no uniform title field is present in the MARC21 record seems to be a serious loss of information. Or perhaps I should put this as a question: in the absence of a unform title element, can we assume that the transcribed title is the appropriate work title?
To conclude, I guess I will go ahead and harp on a common nag of mine, which is that copying data from one serialization to another is not the transformation that will help us move forward. The "work" is very complex; I would feel less concerned if we had a strong and shared concept of what services we want the work to provide in the future, which should help us decide what to do with the messy legacy that we have today.
Footnotes* Note that in 1877 there already was a "Co-operation committee" of the American Library Association, tasked with looking at cooperative cataloging and other tasks. That makes this a 140-year-old tradition.
"Of the standing committees, that on co-operation will probably prove the most important organ of the Association..." (see more at link)
** If you want more about what FRBR is and is not, I will recommend my book "FRBR: Before and After" (open access copy) for an in-depth analysis. If you want less, try my SWIB talk "Mistakes Have Been Made" which gets into FRBR at about 13:00, but you might enjoy the lead-up to that section.
References Wilson, Patrick. Two Kinds of Power : an Essay on Bibliographical Control. University of California Publications: Librarianship. Berkeley, Los Angeles, London: University of California Press, 1978.
 Smiraglia, Richard. The Nature of “a Work”; Implications for the Organization of Knowledge. Lanham: Scarecrow Press, 2001.
 Lubetzky, Seymour. Principles of Cataloging. Final report. Phase I. In: Seymour Lubtezky: writings on the classical art of cataloging. Edited by Elaine Svenonius and Dorothy McGarry. Englewood, CO, Libraries Unlimited. 2001