Revising is the process in which you examine the clarity of content to check if the essay follows a smooth sequence, with all paragraphs logically relating to the theme. During the process of revision, you will check if the introductory paragraph is catchy enough, whether the body paragraphs contain examples to substantiate your stand, and that the conclusion restates your major premise. When you proofread a written piece, what you are looking for are errors of punctuation, grammar and spelling. You seek to eliminate run-on sentences, disagreements between subjects and verbs, and the incorrect usage of commas and semicolons. In other words, your aim is to polish the document to read better.
Revision includes a study of the ideas in your paper, and is therefore best done in the initial stages as you develop content. You will, of course, continue to revise throughout the process of writing, as you change words and the order of sentences to convey your ideas with greater clarity. Proofreading, on the other hand, is best left till the very end, after completion of the revising. After all, there is no point in agonizing over the grammar or punctuation of a sentence you are likely to change.
Of the two processes, revising calls for greater concentration and effort as you seek to enhance the quality of your work. The term "global revision" refers to observing the final picture the essay presents, whereas "sentence-by-sentence revision" aims at ascertaining that every single sentence, paragraph and illustration are relevant to the essay's theme. In comparison, proofreading is a more straightforward process that involves correcting errors in spelling and grammar and improving word choice and sentence variety while considering the intended audience.
Your essay is an expression of your vision and thought process and you are therefore the best person to do the revision. Although someone who reads your essay can offer suggestions about areas that need revising, they cannot do the revision for you. Proofreading, however, has no such constraints; the "Spell Check" on your computer can help, as can friends. In fact, it is often advisable to have someone proofread your writing to catch errors you might have missed.