Lucasta Miller writes about the essay form for The Guardian.

What is an essay? For most people the word conjures up memories of the things you were forced to write at school by people like Miss Peecher the teacher, in Dickens's Our Mutual Friend, whose essays were "always according to rule". Yet in fact the essay is one of the least rule-bound forms in literary history, a genre that can encompass everything from the 600 pages of systematic philosophy in Locke's Essay Concerning Human Understanding to the handful of paragraphs which make up GK Chesterton's freewheeling mini-masterpiece, A Piece of Chalk.