A few years ago, I would have thought that editing, proofreading, and revising were the same thing. i would have have used the terms interchangeably, and I would have been wrong.
Editing, proofreading, and revising are each a separate process that contributes to the finished product in its own way. If you plan to uses the services of an editor, then the definitions below will help you tell said editor what you really want done with you manuscript.
Revising is the reading of your manuscript to organize your thoughts on paper to match the thoughts in your mind. Revising takes place at the level of the sentence, paragraph or higher.
Editing tests each word and phrase to see that it is accurate, appropriate, or necessary, changing the language more than the ideas. Editing is more stylized and mechanical work, taking place at the level of the sentence or word.
Proofreading is checking the manuscript for accuracy and correctness. The last phase of the editing process, proofreading should be completed after the conceptual and stylistic concerns have been addressed. You review spelling, punctuation, capitalization, and usage to make sure no careless mistakes.
As an Author, I find it good practice to revise before you edit. First, in revising you may cut out whole sections of the draft because they no longer suit your manuscript. If you have already edited those now-deleted sections, all that careful work goes to naught. Two, once you have invested time in carefully editing sentences, you become reluctant to cut them, even though these sections may no longer suit your purpose.