Developmental Editing
Developmental Editing looks at the manuscript from the widest angle, the broadest view. It focuses on bigger picture items that often run through the whole manuscript, things like character development, plot, pacing, etc. It also looks at the balance of scenes and chapters: is there too much dialogue? Too little action? Enough exposition? There aren't any edits done within the manuscript; rather, comments and suggestions made to the side of it. These comments are usually directed at things that can be fixed on a larger scale.
Line Editing
Line Editing takes a closer look at the manuscript than Developmental Editing, usually at the paragraph level. It focuses on smoothing out sentences and paragraphs without losing the author’s voice and style. Line editors look for things like redundancy, clarity, strength of diction, and more, seeing how they can elevate these things without changing them completely. They actually make edits as suggestions within the manuscript. Line editors also look at consistency of details within a manuscript. They note if a character was blonde on one page and has brown hair on another, leaving a comment to make the author aware of the discrepancy.
Proofreading
Proofreading is one of the final steps in the editing process, after Copy Editing, and it is the most narrowly focused of the processes. It sticks to the simpler mistakes that one might find in a manuscript: punctuation, spelling, and grammar errors, to name a few. Proofreading is not a copy edit or line edit! If there are problems that seem too overt in the manuscript, they might be commented upon, but they will not be adjusted within the manuscript itself. Even so, proofreaders are judicious with their comments so they don't overload the author. Their primary purpose is to fix minor errors.