Functional Requirements for Bibliographic Records (FRBR)
In a lot of cataloging documentation, including the information in Folgerpedia, you will see references to "manifestations," rather than "books" or "DVDs," etc. This refers to the conceptual model established by FRBR, the Functional Requirements for Bibliographic Records released by IFLA in 1997 which, among other things, sets out four levels of entities: Work, Expression, Manifestation, and Item (often referred to as "WEMI"). FRBR informs much of the reasoning behind cataloging practices and the current usage of MARC.
The FRBR entity levels are not perfect (try to puzzle out where an early modern manuscript fits, for instance, or a music remix). However, they are a helpful framework to begin thinking about what a catalog record should and/or does describe.
The "too long, didn't read" version of FRBR: works are realized through expressions, which are embodied in manifestations, which are exemplified by items. (This is paraphrased from the diagram in Figure 3.1 of FRBR.) You can also skim through Barbara Tillett's brief summary "What is FRBR?" (pdf).
The work is the conceptual idea of the plot of Hamlet as it was thought out by William Shakespeare. (Even though many of Shakespeare's plays have antecedents in folklore and some concurrent writings, they are generally considered to be distinct works on their own.)
The expression is the realization of the work of Hamlet as it was recorded. A work can (and often does) have many different expressions.
The manifestation is often conflated with the edition of a work; it's not a perfect synonym, but it can be a helpful heuristic. The concept of manifestation represents all editions with the same characteristics: this can refer to either intellectual content or physical form (in the sense of the distinction between a text printed in a physical book and an audio recording of that same text; two books could have different cover images, and still be considered the same manifestation, if their inner text is the same). A facsimile of the First Folio published in 1968 is considered a manifestation. A 1920 Croatian edition of Hamlet is considered a manifestation, as is the 1905 Polish edition from which the Croatian edition was translated. Generally, catalogers try to create records for the manifestation of a work (from which other libraries can then note that they also have a copy), but the line between expression and manifestation can be a little blurry.
The item is the physical copy, of whichever version of Hamlet, that the cataloger is holding in their hands (or viewing on a screen, etc.) as they are cataloging. This is known as item-in-hand cataloging - the item is assumed to be a standard representation of its manifestation, and any markings or features obviously unique to the item, such as annotations or a special binding, are recorded in a local note.