I like to look at it as THREE things:
Starting at the very base line of digging into it, there is PROOFING. This is where, on word to word, sentence to sentence level you look for errors of style, grammar, and syntax.
Moving up a bit you have EDITING. This is also a sentence to sentence but also paragraph to paragraph examination of your paper. Are there things you need to add? Are there things you need to remove? Are there things you should move around?
REVISING is the last and most wholistic. This is where you take your thesis statement or purpose for a paper and look at the paper as a whole.