If copy editing is nitpicking the little errors and developmental editing is focused on the big picture, then line editing is right in between. A line edit is mostly concerned with the effectiveness and correctness of the prose. If your story is interesting, but the way it's conveyed is boring or unclear, you need a line edit. What you can expect if you contract me for a line edit is corrections of egregious language errors, in-text comments in areas where I see the need for improvement (but it would be inappropriate for an editor to re-write) and a running commentary on strengths and weaknesses of the language used in the manuscript. I may or may not include a narrative write-up at the end, depending on whether or not I feel it is necessary.