Once a book is completed and edited, proofs, galleys, or advanced reader copies (ARCs) are typically distributed to book buyers, reviewers, book sellers, sales representatives, and magazines to assist in the marketing and sale of the work. Both advanced proofs and galleys will typically include a summary, author biography, and helpful marketing information on the back cover, and they are released three to six months in advance of publication.
Though the terms ARC, proof, and galley are often used interchangeably, at one time there were distinct differences between them. Galleys were only used in the editing process and were typically low quality black and white photocopies. Proofs were typeset copies that may or may not have artwork that were distributed to reviewers and booksellers to promote the book. ARCs were finished copies that were sent out early for promotion purposes.
Today the differences between the three terms are less clear. The term ARC now appears to be used as a synonym for a proof, leaving only galleys distinguishable because they are typically printed in black and white and without any artwork.
There is a thriving market online for galleys and proofs, which frustrates many authors since edits might still occur before the book is released. These edits might be limited to spelling, but there could also be factual errors that are fixed or substantive editorial changes. In addition, authors don't receive any royalties for the sale of the galleys and ARCs.