The editing services I provide fall into three categories: proofreading, copyediting, and substantive editing and rewriting.
If words were wood and the finished product a chair, substantive editing would fit leg to side rail, arm to back, tenon to mortice; copyediting would strengthen loose joints and set plugs; and proofreading would give the final sanding and lacquer.
Every manuscript needs to be copyedited before it’s published. Every manuscript needs to be copyedited. “Proofreading” is not a panacea. You may not see the rough edges on your manuscript: the nonstandard punctuation, the confusingly paragraphed dialogue, the unintentional humor of a misused word, the use of five words where one would suffice. View an example copyedit.
Your particular manuscript might also benefit from substantive editing, which needs to be done before the manuscript is copyedited to avoid duplicate work. If your book is to be printed, it might be good to have the final pages proofread before the file goes to the printer, especially if there were many changes during copyediting.
The Stages of Editing page explains what happens to your manuscript during the various steps.
Please note that there are no industry-wide standard editing definitions. What I call “substantive editing and rewriting” is called “developmental editing” by the Bay Area Editors’ Forum and “structural and stylistic editing” by the Editors’ Association of Canada.