What's the difference between editing and proofreading?
And is copyediting something different from those?
Editing is looking at the sum total of a story or article, and improving content -- flow, word choice, plot, pacing, revising and cutting and reflowing as needed to make the work better. By its nature, editing involves things that aren't black and white, right or wrong, but it involves value judgments on the current state of the copy that's written, judgments from the perspective of someone whose opinion you trust.
Copyediting is technically the same thing as editing, but sometimes some people reserve copyediting for nonfiction and editing for fiction. It's not the norm, but I've seen that described that way.
If you want more information about proofreading and editing, the best references I've seen include:
The Chicago Manual of Style (for anything)
The AP Stylebook (for articles and nonfiction)
The Microsoft Manual of Style (for anything with a tech slant)
Strunk & White (for a basic overview of grammar and usage)