It used to be that when I asked people to correct my writing, I’d get confused about whether to ask them to “proofread” or “copyedit.” No more! Now that I’m in the publishing industry, I actually know the difference.
“Proofreading” is performed on proofs—in essence, the rehearsal round of a book or other manuscript that’s about to be published. Proofs are already as perfect as you can make them, and physically resemble the finished product. Proofreading is going through the text, line by line, to spot whatever small errors are left (or were introduced during typesetting).
Copyediting happens much earlier in the process, and involves checking writing for problems with clarity, grammar, style, and so on—in other words, cleaning up the writer’s final draft. Generally, this process will generate queries that must be resolved with the author; proofreading, in contrast, rarely does.
Or, as the gentleman who taught my manuscript editing class, Erik Carlson, told us, proofreading can be done with music on. Copyediting can’t. These days I request that people copyedit my resume and cover letter.