A pre-first edition is any limited run copy of a book printed before the first standard edition is published. These include advance(d) reader copies (ARCs), galleys, salesman’s editions, proofs and sometimes manuscripts. The importance of these editions varies from collector to collector - some enjoy seeing the evolution of a book from concept to publication while others are only interested in collecting the editions meant for public release.
Here are a few different examples of pre-first editions that you might encounter, and remember that these terms are loose definitions. Some book dealers and collectors use the term manuscript and galley interchangeably, but others consider a galley and an advanced reader copy the same so make sure you know what you are buying.
Types of pre-first editions
A manuscript is usually the first iteration of a book's text as it was conceived by the author. A manuscript would usually be a copy which pre-dates an uncorrected proof or galley. These are often marked, unbound and sometimes handwritten.
Most often they are one-of-a-kind, but may have been copied for a small number of individuals. To some collectors, a manuscript is not considered a pre-first but rather a particular kind of ephemera.
These are preliminary versions of a publication meant for review by the author, editors and others within the publishing house. Galley proofs (often simply called galleys) may be unbound, uncut (where the pages have not been shaved down to a uniform surface) or electronic in the modern era of publishing. The term galley proof comes from the days of hand-set typography where the metal trays used to tighten and set the type into place were called galleys.
Advanced reader copies
Also known as ARCs, or advance review copies, these are produced privately by publishers and distributed to booksellers and journalists prior to the official release date. Because ARCs may not have been put through the entire editing process, the copy will often differ slightly from the standard edition of the book. ARCs may also have different cover art.
Sometimes called a salesman's dummy or publisher's dummy, these books look exactly like the final consumer edition except they only contain a small amount of text (perhaps only the first chapter). These books were used by the publisher's sales force to help convince bookstores to carry a forthcoming release.