A developmental editor helps you shape your book. Rather than focusing on a line-by-line edit (though some developmental editors also do this), the goal is to focus on the structural organization of the book – does the narrative flow coherently? Are there plot holes? Are terms explained appropriately for the audience?
A developmental editor will help you trim areas of your book that are too long, and punch up details that need highlighting. They are sometimes called “book doctors”.
But do you really need a developmental editor?
Most books need another set of eyes to objectively review the content. It’s a rare author who can write a perfect book without feedback. A developmental editor doesn’t re-write the book, but helps its evolution, in the writer’s own voice.
Most of us, even English majors, make grammatical mistakes. The difference between a copy-edited book and one that has not been copy-edited is enormous. Copy-editing doesn’t change the substance of what you’re writing about. In fact, it enhances it – clarifying meaning, correcting distracting mistakes.
A good copy editor will adjust your punctuation and spelling, question whether or not you really want to use jargon, make sure you’re using the right terminology, and keep you from embarrassing errors of usage. He will keep your language consistent from page to page, and ensure that you capitalize names properly.