Developmental Editing - You have an idea for a book and would like to write it yourself, but you need coaching, someone to work alongside you to help you flesh out plot, characterization, and ideas.
Content Editing - You have a completed book you plan to publish, but you need a skilled pair of eyes to read it, tell you what areas need work. Maybe it's characterization. Maybe the plot is too thin. A content editor will read, report back to you, and work with you to get it just right.
Copyediting / Proofreading - When many people think of editing, what they really think of is a copyeditor or proofreader. If what you need is someone to catch your spelling, punctuation, and formatting errors, you need a copyeditor/proofreader.