A critical edition is a scholarly publication that reproduces the text of a historical document in such a way that it is as close to the "original" as possible. Any differences between surviving versions of the text in question will be footnoted for the benefit of scholars who wish to compare them. To save space, each surviving copy of a document might be given a code (called a siglum); the differences between them will be listed in the apparatus criticus, which is to say the technical footnotes to the body text. The body text itself is supposedly the "best" version of the document -- at least in the opinion of the historian who prepared the critical edition. The introduction to the work is where she will explain why she made the decision to value certain versions over others. These decisions, needless to say, can be extremely controversial.

There are three main circumstances in which someone might want to create or consult a critical edition.

• Ancient manuscripts, by definition, are written by hand, and tend to differ from one another in significant ways due to scribal errors, regional writing conventions, and the organic growth of literary and philosophical traditions. The most famous example of a text that survives in multiple versions is the New Testament, which varies considerably across the 2,000 or so surviving manuscript fragments. A critical edition of the entire New Testament would be impossible to fit into a single book, or even on a single shelf for that matter, though certain computerized versions try to create hypertext editions that account for as many variations as possible. Some editors work on critical editions with a more modest scope; these may represent only one specific manuscript, or family of manuscripts, rather than the entire textual history of the Bible. Even these more restricted projects can run to thousands of pages.
• The vagaries of handwritten texts lead to a lot of difficulties for the ancient historian, but texts could vary quite a bit across editions after the invention of the printing press as well. For instance, think about the biology textbook you used as an undergraduate: it's likely that the cover said something like "Seventh Edition" beneath the title. The reason for this is that the nature of scientific knowledge is changing all the time, and books need to be updated to match the needs of the contemporary student body. A cultural historian or a historian of science might find these changes very interesting: he could use them to map shifting attitudes toward, say, sexual reproduction in the culture that created that textbook. Comparing the differences between versions of a scientific, religious, or legal text is easier when one has a critical edition at hand.
• Finally, individual writers often tinker with their own work during or after the publication process. Some poets, like Emily Dickinson and Gerard Manley Hopkins, obsessively rewrote their poetry, and it is not always obvious which version of their own work they preferred. The editor of a critical edition will compare, not only published versions of the work, but the author's journals, letters, and working notes, with a view to creating a version of the text that best reflects the author's "original" or "ideal" vision, if indeed it's even possible to talk about such a thing.

For the casual reader, a critical edition is usually not necessary and can in fact be confusing or distracting. For day-to-day use, an eclectic edition -- which is to say a publication that has been cobbled together out of multiple versions to make a sort of "average" -- is generally most convenient. Most store-bought Bibles are eclectic in this sense.

If a critical edition isn't hardcore enough for you, you might be in the market for a facsimile edition, which is a photographic reproduction of a document.