Many different conventions exist for the form an essay or text should take. Writers should be careful to work within or against those conventions with purpose and expertise. [EDIT]
Whether the writer is a student in a course or a scholar submitting a work for publication, the writer must be aware of the essay conventions guiding their work.
We can generally assume that essays must be multi-paragraph and that they should have a solid beginning, middle, and end. However, even those guidelines look different from one setting to the next. Careful writers should pay close attention to paragraphing and organizational structures when writing essays. Further, writers must be aware of the expectations for any essays they are submitting for a grade or publication.
For a resource addressing a traditional view of the essay form, see the following link:
Students and scholars would be better served reading a wide variety of nonfiction and scholarly essays in order to develop a more sophisticated view of essays.