There are three basic types of editors (and they each have various names). Each type of editor requires a different skill set.
A developmental editor, sometimes called a comprehensive editor, looks at big picture issues. For fiction this includes items such as story arc, character development, writing voice, and plot issues. Nonfiction looks at theme, organization, structure, writing consistency, and so forth.
A developmental editor must read widely and have knowledge of your genre and the publishing industry.
A copy editor looks at sentence structure and the flow between sentences, paragraphs, and chapters. They will identify awkward sections and poor phrasing. They may point out character inconsistencies and possible factual errors to check.
A copy editor needs to know the genre. Having a college writing degree helps, but a more beneficial characteristic is having taught writing and graded a lot of papers or has experience in a career that requires a lot of editing.
A proofreader looks at the details: word usage, punctuation, and grammar. A proofreader should enjoy specificity and be able to focus. A proofreader must know and follow a style guide, such as the Chicago Manual of Style (CMS).
Some proofreaders know multiple style guides, but others specialize in using just one and therefore only take jobs that use that style guide. The key requirement is having mastered a style guide and knowing how to apply it.
No one can do all three types of editing at once—nor should they. And most editors will only ever do one type.
The ultimate qualification to become an editor is having successfully done the work. This makes it hard for people to start as an editor because few writers will hire an unproven editor.
Peter Lyle DeHaan, PhD, is an author, blogger, and publisher with over 30 years of writing and publishing experience. Check out his book The Successful Author for insider tips and insights.