If you have ever looked into editing services it can be confusing.
There are basically four types of editing:
1) Developmental/Substantive editing:
This type of editing looks at your writing as a whole. This is the first type of editing that should be done.
Substantive changes include things like reorganizing paragraphs, sections, or even chapters to improve how the story is told. You might be asked to delete a whole section or add something to make the story flow better for the reader. I also look for gaps in the story or with consistency with plot, tone, characterization… and things that don’t belong or don’t move the story forward.
When I review books, this is the type of thing I will pick up on first thing. I recommend you give your book to at least two friends who you know like to read. Someone with a readers eye can pick up on a lot of these things for you and you may not even need to hire someone at this point.
Note: some people don’t even need a developmental edit. Your editor will let you know once they start reading your manuscript.
This type of editing is on the sentence-level, looking for problems with grammar, spelling, and punctuation but also consistency with style, story and word usage, good transitions, dialogue attribution, sentence parallelism, wordiness (sentences that are too long or not as “tight” as they could be), too much passive voice, and overuse of adjectives. Clarity is paramount; The text has to be understandable.
For most fiction works, The Chicago Manual of Style is the reference of choice. For non-fiction, it could be the AP Style Manual.
The copy editor can also look at accuracy. They may ask you to check a certain person, place, or thing you reference for accuracy. This is important in any writing, fiction or nonfiction alike. No one loses interest in a story faster then when they know the author doesn’t have their facts correct, even if it’s a fictional story.
Some editors may divide their editing into Light, Medium, or Heavy. I don’t do this because if I see something wrong with a manuscript, I want to help fix it, not just leave it because I said I was only going to look at certain aspects of the writing.
I have a certificate in copyediting and have done additional education in children’s book editing.
3) Line editing
Think of this as more of a style edit, the way you use your words and language to convey meaning.
It is considered a sentence level edit but not looking for the same errors as in copyediting (or mostly not the same). It can address syntax/sentence structure, tone, word choice, voice, dialogue, transitions, pacing and consistency.
It might be thought of somewhere between a copyedit and a developmental edit (sort of).
This type of edit costs a bit more because it takes more time to do and takes more skill.
This is the last stage for your book and where it is checked for errors before it goes off to the printer or e-publishing company. I will look for spelling or punctuation errors that fell through the cracks in the copyediting phase, but also make sure the titles, page numbers, header, footer, paragraphs are correct (no widows or orphans or too many hyphenations at the end of a line of text) and the book meets layout and styling specs. I think this blog post does a great job of explaining how proofing is different than other types of edits: https://ckbookspublishing.com/2013/11/04/proofreading-explained/
As part of my copyediting certificate, I did extra training in proof reading, plus I’ve done extra proofing education since.