Funny, how things have evolved. "Galley Proof," which is the term for the copy-edited manuscript sent to the author and editor for approval, is an old printer's term, comes from the oblong shape of the tray used to hold moveable type.
(And I love it that a manuscript they are done with once the book is in print is known as "foul matter ...")
When I started in the biz, galleys came back as one of the mss copies I submitted, with editor's and copy editor's marks on it, usually in colored pencil. Words that were misspelled, used wrong, punctuation, awkward compositions, all like that, were circled, lined through, given proof reader's marks, which you were expected to know and use, and sent back for me to deal with.
Other minor revisions going to sgory content usually came in a cover letter.
Major revisions were addressed in a separate cover letter, usually following a phone call with the editor. Luckily, I haven't gotten many of these.
Proof reader's marks are interesting.
If there was an unintended space, then the delete mark, which looked kind of like a curly French fry, and a pair of arcs, above and below, were used to tell the printer to close the space. Italics, capital letters, insert punctuation or letters, hyphens, em- or en-dashes, all had their own marks, and you can find those in most style manuals, or just by searching the web. Or by clicking on the illustration above to make it larger.
You don't need to do that, though, since like buggy whips and typewriters, editing has moved along, and CE'ed manuscripts are now mostly done electronically. My most recent couple of books were edited using Word's Track Changes and Notes functions.
How this works is, the editor or CE changes this or that, and it shows up in the left hand margin, telling readers what was done–e.g. deleted or added words, etc., with the changes showing up in a different color for everybody who fools with the manuscript.
The yellow notes will ask questions–Did you mean this or that? This name doesn't agree with one you used earlier. What does this term mean?
The writer can agree or disagree or explain as necessary. Save it, attach it to an email, and poof! it's gone. No more foul matter, since there isn't a paper ms on either end. I submitted this book in as an electronic file, and it won't exist on paper until it is done and printed for the racks.
Welcome to the future ...