Content editing (also called "structural editing" or "developmental editing") takes into account the entire text, whether it's a book, sales brochure, website or other content. When editing for content, the editor seeks to ensure the logical flow or correct sequencing of ideas; consistency of the writer's voice; consistency of character (such as in a novel) and other broad areas.
The editor won't always take it upon him/herself to write the changes (unless it has been agreed up with the client ahead of time) but will make notes, either directly into the text (in a contrasting color) or by use of margin notes, such as the "Comments" function in Microsoft Word.
Content editing is not always a matter of removing content or re-arranging it; it can also be a matter of what I call "the canyon effect." The person who wrote the copy leaped from one idea to another without a proper transition. So the reader, missing the transitional material, does not make a smooth transition; they fall into the canyon. I note this in particular with technology-related material, written by people who are tech-savvy but who are not writers by trade.
While it probably goes without saying, content editing isn't just for novels or other books: I have had clients request it for two-page articles and tri-fold business brochures and other "smaller" jobs.