I'm an editor with several years of experience. I've done freelance editing, and I've also worked for publishers. In both cases, I was typically the last person to get my hands on books before they went to print, and my job was to work with the authors to make sure the stories they had worked so hard on were ready to go to publication.
I've also been on the other side of things, so I know what it's like to go through the editing process as a writer. I've been published through a small publisher, I've self-published, and I worked with a literary agent for several years. I know what it's like to write a book and to pour your heart and emotions into the story and characters. After all the sweat and tears, late nights and early mornings, joys and triumphs, and maybe a few too many caffeinated beverages, it can be very hard to turn all of that hard work over to an editor for scrutiny.
When I work with an author, I do both content and line editing. I look for inconsistencies, both in plot and in little things. For example, I pay attention to details such as why a character who was just sitting on a sofa is now, several paragraphs later, suddenly sitting at a desk. I correct grammar and spelling, offer suggestions on how to smooth out clunky prose, watch out for redundancies in word usage, make sure wording is clear, and check that there aren't plot holes--all without disrupting an author's unique voice and style.
Every editor has their own style, and mine is to try to help writers become better editors themselves. If I see a consistent grammar mistake, I'll point it out and explain the reason why I'm correcting it, so that when the author writes another next book, their writing will be even more improved. I love watching writers grow in their craft, and I find it especially exciting when I get to work with an author on more than one book and see how their writing continues to develop.