There has been some debate in our office as to what services should be covered under the category of "editing", and what our clients should expect if they need editing as opposed to proofreading. In fact, there are generally three levels of editing work, only one of which is what's commonly called proofreading.
In reading Lillie Ammann's series on self-editing, I came across a short and concise list summarising the different kinds of editing functions that I think are applicable to all editing of the kinds we provide at our bureau.
I reproduce it here:
Content Editing Also called developmental, substantive, or structural editing; revising; rewriting
Revising or moving entire paragraphs or sentences
Adding new material to fill in gaps and deleting original material that doesn’t work
Re-organizing and restructuring content to improve flow and clarity
Copyediting
Also called line, mechanical, or stylistic editing
Correcting spelling, grammar, punctuation, and mechanics
Checking that the content follows the appropriate style guide or internal style sheet
Verifying facts and ensuring consistency
Clarifying meaning and improving readability by changing word choices and sentence structure.
Proofreading
Reading the final copy of the manuscript to check for errors
Ensuring that all changes have been incorporated and that no errors have slipped in during the editing process
While I don't 100% agree with the labels suggested here, it is indeed the case that there are three levels in the editing process: heavy, light, and proofing, which correspond to the three descriptions given above.
When you need some editing work done, it's always helpful first to decide what kind of editing you're looking for, so that your hired editor doesn't either tread too lightly or otherwise crush your work with unexpectedly heavy rewrites!