inkycompass answered: What it’s like to have your work professionally edited!
Well, inkycompass, first of all I don't know whether I've been unclear elsewhere, but I haven't had an editor at a publishing house edit my work, if that's what you mean. That will happen when I sell one of my books, and I'll be sure to answer more comprehensively when I have something to say about my personal experience with it. But! I do still have insight. In three parts!
One: Professional-level editing advice I have received
Though I haven't gone through edits with a mainstream publisher, some of the people who have looked at and commented upon my work have been publishing industry professionals. More than one of my beta readers had edited professionally, and their edits were usually more sophisticated than the edits of my other readers. They tended to use the Microsoft Word features a lot more, with comments and cross-outs/word substitutions as well as justifications for their changes.
I've also had my work commented upon by my agents, of course, and though both of them provide editing tweaks for their clients where necessary, neither had much to say on my manuscripts/proposals. I believe my fiction agent is more of a hands-on editor than my nonfiction agent, because she comes from an editing background, but she said mine was unusually clean and she didn't have editing suggestions for me. My nonfiction agent had editorial feedback to the tune of moving sections of my proposal around, and also complimented my proposal for being well-written.
And the publishers who have come back with negative feedback so far have usually rejected for some content-related reason (lack of personal connection with the story, had a similar title already, etc.), and some even complimented the writing, so I have yet to get any specific style or technical editing comments on my writing from publishers.
Two: Professional-level editing advice I have dispensed
I won't go into a lot of detail here, but what it's like when I edit a person's writing professionally is, well, pretty comprehensive.
That is, if the author is ready for that.
Sometimes I just send back a piece with bits and pieces of pluses and minuses with a couple spelling corrections. If you get this from me, I probably think it's not ready for a scouring, because I only get really picky on things that seem like they could really be good if those things got cleaned up. When I edit someone's manuscript, I comment on the style and grammar and spelling with corrections, and I also usually provide comments, talk back to characters, ask questions, and say what I liked (so the author will know what is already working).
Three: What I know about the professional editing process: edit letter and line edits
But I do know what I'm in for when I get a contract and have to deal with the next (oh-so-fun) part. Once you have your book contract and release date and whatnot, the editor you're working with sends you an edit letter. You receive an explanation of how they would like you to use your edits (how to save the file and respond to comments/questions/edits) and probably a deadline. You get a specific stylesheet that explains how that publisher wants certain things (like, what's hyphenated, how numbers are written, capitalization conventions) and what they've changed in your manuscript. And then when you're through with this edit (or possibly a couple rounds of this), you get copyedits and line edits. You'll have to respond several times, and much of this process is really determined by how your publisher works. I once did the proofreading on a book when it was in a ridiculous unbound hard copy:
The Road to Publication Part Four
The Road to Publication Part Five
The Road to Publication Part Six
The Road to Publication Part Seven
Jill's journey was a little different because she won a contest and her timeframe was way faster, but it's still got some interesting stuff about working with a mainstream publisher on the editing phase.
I hope that gives you a decent perspective even though it's from someone who hasn't personally been through it yet!