Sometimes also called Structural Editing.
This is the first stage of editing and it's all about the big picture - the plot (and plot holes), character arcs, romance arcs, worldbuilding, pacing, logic and consistency (e.g. a character puts on red shoes at the beginning of the scene and suddenly they are blue). It can also cover choice of POV, person and tense.
This kind of editing focuses on the plot and storytelling, not on the perfect wording or correct grammar. There's no use in correcting commas in a scene that's going to be rewritten or deleted.
Sometimes also called Substantive Editing.
This is all about finding the right words and the perfect rhythm. The editor will make suggestions at paragraph and sentence level on word choices, takes care of repetition and redundancies, unclear writing and improving the flow.
Line editing often goes hand in hand with copy editing.
Sometimes mistakenly called proofreading. (It's not the same!)
This is all about the technical correction of your manuscript. In this step, the editor will correct spelling, grammar and punctuation errors. They also look for consistency in style choices (e.g. spellings that are correct in more than one way, but should be consistent throughout) and create a style-sheet where all those choices are listed. This is extra important when you write a series.
Copy editing also includes things that often go unnoticed, some of them being:
- placing correct typographical special characters e.g. for an ellipsis,
- choosing the right hyphens or dashes,
- making sure all your fonts and sizes are consistent throughout the manuscript and
- that the page numbers of the chapter beginnings match the index.
The final pair of eyes.
This happens ideally after the formatting - which is especially relevant when you want a printed edition of your book.
The proofreader not only checks for spelling or punctuation errors that slipped through, but also has an eye on possible issues caused by formatting, such as single words or lines at the beginning or end of a page (known as "widows" and "orphans").
For best results the final proofread should always be done by someone who hasn't worked on your book before.