At its most extreme, revising means discarding a draft and starting over. In this extreme of revising, the writer rethinks the writing, changing content as well as organization and style. At this level of revising, the writer is concerned with the basic content, the concepts and fundamental imagery of the writing.
A less extreme form of revising means reorganizing the draft and adding or removing sections of it. Revising at this level, the writer seeks to clarify content and style and to make the writing more accessible to the reader. At this level of revising, the writer is concerned with the overall structure of the writing, making it express the basic concept of the writing.
A third level of revising is close to proofreading. At this level, the writer mainly checks sentence and paragraph style, not so much looking for errors as seeking clear and effective writing. At this level, the writer is largely concerned with the rhythm of the writing.
Remember that, like other writing processes, the levels of revising can overlap and repeat as the writer works. Writing can be a messy process.