An essay is a literary composition that expresses a certain idea, claim, or concept and backs it up with supporting statements. It will follow a logical pattern, to include an introductory paragraph (make the claim), a body (support), and a conclusion (summary of statements and support).
English and Literature teachers use them on a regular basis, but essays are required in many other fields. Essay exams are also a test tool used commonly in the social sciences, and even in math and science class.