If you are anything like me, you have once in your life thought that "proofreading" and "copy editing" are synonymous words, and thus, interchangeable. I could not tell you how surprised I was when I learned, during my senior publishing lab at ISU, that the two are drastically different. Who'da thunk, right?
As a writer and an editor, it is vitally important to both acknowledge and understand that these terms are different, what each one describes, and how each one is performed.
Proofreading is a basic form of editing, meant for catching errors in typography (typos), misspellings, incorrect/missing punctuation, or errors in grammar. Your editor will call your attention to these conventional errors by using proofreader's marks, normally in a bright color that catches the eye (red is most common.) Your editor will read through your draft once, and then they should read through it again to confirm their corrections or catch any additional errors they may have missed the first time around. It doesn't hurt to ask your editor to read your manuscript twice in proofreading. Normally, an editor will specify which style guide he or she uses when questions arise. For example, I use The Chicago Manual of Style. In proofreading, you editor will not comment on the plot or content of the manuscript at all, and there are rarely any queries.
Copy editing is a more in-depth and time consuming venture for both the author and editor. It will take longer to get a manuscript back if it is submitted to an editor for copy editing. In copy editing, it is the editor's job to fulfill what is known as the "Five Cs", that is, that the document is clear, correct, concise, complete, and consistent. An editor will examine the content of the manuscript, comment on errors in consistency (plot and language), style, unnecessary or missing words or information, flow, and accuracy. Copy editing involves lots of queries, usually in the form of Post-It notes. If the editor has a question for the author, he or she will indicate where this question arose and what it is in a separate note to the author. Your editor may also ask you to clarify, expand on something, or omit confusing or conflicting information.
Why it is important:
As a writer, you want to make sure you understand which service is which, and which one will benefit your manuscript the most so you can give your editor correct instructions, and ultimately end up getting what you pay for.
If you are in the early stages, or are not totally confident in your work, then copy editing will be a better choice. Your editor will go through your manuscript with a fine-tooth comb, observing inconsistencies, plot holes, incorrect facts, items that don't make sense, conventions, grammar, and style. This will be especially important if you are a stream-of-consciousness writer or have a complex storyline or idea.
If you have gone over your work many times, or are in the later stages (sending sample chapters, or even querying agents or publishers) proofreading will be all you need. You've gotten a good hold on your content and story, but just need a second set of eyes to make sure all of your commas are in place and that nothing funky happened to your spacing. Another set of eyes will catch errors you may have overlooked when correcting your more substantial errors in your later drafts.
And obviously, an editor needs to know the difference so that they can complete the service that they are hired to do. A happy client is a repeat client, after all!