The word “editing” has acquired a whole raft of different meanings over the years, not all of them accurate.
A commissioning editor finds books which she considers worth publishing, then commissions them—that is, she contracts the writer, and everyone else required to produce the book.
She then works with the writer to refine the text until it is as good as it can possibly be. This involves working on the structure of the book; ensuring that the tense and point of view are consistent and appropriate throughout; that all facts are correct, no one is libelled, no one is misquoted and no work is plagiarised. Slower passages might be reworked or deleted; in the case of fiction, plot-holes are filled; a few subsidiary characters might be amalgamated into one, or weak characters deleted entirely. All this takes time, and can be painful for an inexperienced writer, but in just about every case it results in a book which is tighter, cleaner, more fluid—and much more readable.
Once a text is arrived at which the editor and the writer are both happy with, the text is handed over to a copy editor who checks it again, this time on a more basic level. The copy editor looks for errors in spelling, grammar and punctuation, checks that characters’ names and locations are spelled consistently throughout, and ensures that no confusion exists between words like it’s and its, discreet and discrete, and there, their, and they’re.
Once the copyedit is complete the writer is given the opportunity to check that the changes meet with her approval then the text is handed over to the designers, who determine its layout and appearance. At this stage, it used to be commonplace to print out galley proofs and give the writer one last opportunity to check for errors (which is where the term “proof-reading” comes from). Increasingly often, though, as publishers cuts costs, galleys are not produced and texts receive just two levels of editing before they go to print.