An essay is fairly brief. While writers will sometimes refer to book-length texts as "essays," the term usually refers to short pieces that might be published in a magazine or newspaper.
An essay is nonfiction. That means that the writer of an essay is trying to tell the truth, not merely entertain. A short story isn't an essay because it's fiction. To say that an essay is nonfiction doesn't mean that every word of an essay must be literally true. Hypothetical examples can be an effective way of illustrating a point, as can quotations from fictional works. But such imaginative illustrations should always serve the purpose of clarifying or illustrating a claim that the writer believes to be actually true, in the real world.
An essay tries to make a point. This is perhaps the most important and most challenging aspect of the essay. An essay is not just a bunch of words, or even a bunch of paragraphs. An essay all fits together; it all points in one direction. An essay leads to one conclusion. This is what makes an essay different from, say, an article in an encyclopedia, which may be a relatively brief and interesting piece of