What Exactly Is a Copy Editor?
http://www.theslot.com/copyeditors.html

Walsh, Bill
Slot, The
2002

Abstract:

Copy editors check written material, usually as the final step before it is set into type, to correct errors in grammar, spelling, usage and style (in this case, style refers to a given publication's guidelines for consistency in how words, phrases, typographical elements, etc., are to be used -- or not used). Copy editors are not proofreaders, although reading proofs is often part of the job description. The difference is that proofreaders (a job title that scarcely exists anymore) are charged with simply looking for typographical and mechanical errors on copy that has already been typeset. Proofreaders -- and, indeed, copy editors reading proofs -- are often criticized rather than praised for making picky changes at that stage in the process, whereas the same changes might well be applauded at the copy-editing stage.