There are three stages or parts to the editing process: structural editing, line editing, proofreading and copyediting.
A structural edit is the first edit. It tells the author what to keep in and what to take out. This edit helps the writer to get the plot in place and make sure that it is consistent throughout. A structural edit will also deal with Point of View, Show not Tell and Characterisation. A writer may have been attending a writing group or critiquing sessions in which some of these issues have been dealt with as the writer has progressed with the story.
A line edit is more concerned with how the story reads line by line. A line editor is concerned with how each line serves the character, the plot and the Point of View. It is not concerned with the overall structure of the story.
Proofreading and copyediting look at the written words in terms of grammar, spelling, factual inconsistencies and typos. Copyediting and proofreading should, ideally, be done before the manuscript is sent off to t a line editor or structural editor.
The editing process can be an expensive business for a writer, so the writer should rely on readers and writing groups as much as possible to reduce the amount of work an editor needs to do. If a writer lands a book contract then the editing is done by the publishing house. Self-publishers may have to do as much of their own editing as they can. A good self-editor might be able to get away with only paying for a structural edit.
All writers need a good editor as a good editor which help the writer develop the book in the best way possible. A poor editor could destroy both the book and the author’s morale. So choose wisely and get the editor to do what you need them to do.