I freelance as an editor and have worked on several books, both for publishers and individual authors. Editing a text is about arranging it logically, making sure the chapters and paragraphs are in order, making suggestions for improvement when necessary and supporting the author in creating the perfect text. Sometimes I also lend a hand in the actual writing.
If you have a text that is "finished" but needs a second pair of eyes, then you want proofreading or revision. If you're still writing the text and need help on the way, then you want an editor. In both cases, you want me.