Revising entails ensuring your writing follows a linear process and that the content builds on itself. Your sentences should be clear and concise, your paragraphs should logically relate to the theme and your conclusion should restate the major premise. Proofreading, on the other hand, involves searching for errors relating to grammar, punctuation and spelling. You need to fix misplaced commas, run-on sentences and subject and verb disagreements.
Revision is an iterative process that occurs multiple times during writing. Some writers prefer to create a first draft, a second draft and on so, while others favor revising their writing after each paragraph. Proofreading tends to be saved until the very end, after the revisions have been completed, because it doesn't affect the quality of a written piece as much as the actual content.
Proofreading a piece is much simpler than revising it because it requires simply following pre-established, objective rules concerning grammar and punctuation. Revision is far more subjective, in that there is no right answer. This is why many famous writers have different styles. The way you express yourself may be different than say William Shakespeare or Charles Dickens, but that doesn't necessarily make it any less meaningful.
Proofreading can be partially automated through the use of computer spelling and grammar checkers. Mind you, the end result may still require a few adjustments. Revisions cannot be automated. You must either revise your work or hire someone else, such as an editor, to do it for you.