There are some obvious differences: an essay is relatively short – usually 1500 to 2500 words – and you are told clearly what to do by someone else. For example: Describe and evaluate major theories of globalisation. A dissertation is a subject you chose for yourself. The first usage of the word in the English language in 1651 also gives a useful starting definition: “an extended written treatment of a subject”.
What does the word ‘debate’ imply? A discussion involving different points of view or sets of ideas. A dissertation will therefore not only examine a subject but will review different points of view about that subject. Here’s another definition that underlines some more important characteristics of a dissertation: “a substantial paper that is typically based on original research and that gives evidence of the candidate’s mastery both of her own subject and of scholarly method.”