The Difference Between Editing and Proofreading, Theoretically
Sometimes when I am applying to various low end jobs I see a lot of interchange between the terms editing, and proofreading. (Occasionally I even see editing used synonymously with writing, which leads to miscommunication.)
Proofreading is moreso strictly looking for grammar and spelling errors. Grammar may encompass some clarity issues, but usually, there's either a rule, or there's not. If I'm proofreading, I'm not looking for whether your statements are true, in the best order possibly, or whether all of your sources check out.
Editing, however, is the deeper cut into your work. The more I edit, the more I find it's actually beneficial to do this before proofreading, as sometimes a lot of unnecessary work occurs; if I'm going to delete a particular paragraph, I don't need to check it for spelling and grammar!
Editing may include proofreading, but it is so much more than just looking for surface errors. Editing looks at the actual content and puts everything to the test.
It is beneficial to understand that editing is the part that may offend the writer, if they don't understand what the goal of it is, because it's during the editing process that questions are asked, and doubts may race through the writer's mind about whether their work is good.
If the writer can take a step back and realize that their readers may have the same questions, and that the editor is only doing his job of making the writer's voice heard clearly and their message come through concisely, then the writer can set out to learn from the process and become confident in his work.
So proofread, edit, read, re-read, and do it all over again - knowing that writing is a process, not a quick and easy thing.