An essay is a written text. It is usually about the personal point of view of the author who wrote it. The definition of an essay is vague. Articles and short stories can be quite similar. Almost all modern essays are written in prose, but works in verse have been called essays (e.g. Alexander Pope's An Essay on Criticism and An Essay on Man). People that write essays are called essayists.
Essays can have all sorts of purposes. For example, an essay can be very argumentative, it can talk about points for and against the essay question to give a balanced argument or opinion.