Checking for typos, grammar mistakes, bad punctuation and formatting flaws. This is for writers who are sure that their content is what they want but need a second pair of eyes to look it over.
Making sure the text is grammatical, correctly spelled, readable, consistent in style and factually correct. The editor takes a deeper look at your work than with proofreading.
Ensuring that the work is well organised, makes the most of the material, and makes sense—the deepest, most difficult and most expensive form of editing. This is the one to go for if you are unsure of yourself as a writer.