Editing is the methodology of modifying the first draft stage of any written text, the intention being to produce a coherent and consistently complete work.
In the case of publishing, say as in the case of a book, the starting point lies with the author. In other cases the “owner” of the work could be a marketing manager, art director, or a small business.
The editor may also fulfil one or both of the roles below dependent on the project.
Copy editor — dealing primarily with spelling, grammar, punctuation, fact checking, and word choice. See copy editing.
Development editor — helps the author or owner of the text from first concept to the final draft; suggesting topics, planning the structure, and working through successive drafts with the writer / owner.
What a good editor needs in addition to technical “know how” is an understanding of what makes a project “tick”. As such, editing involves creative skills, soft skills, and a precise set of methods. A great sense of humour goes a long way too.