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”.
Another useful clue is found in the Latin origin of the word – dissertation comes from a Latin word ‘dissertare’ = ‘to debate’.
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.”
A dissertation will show that the writer knows her subject, the key facts and different points of view in it – but it also advances a point of view resulting from original research. Remember that ‘original’ does not mean ‘something that’s never been done before’ but rather ‘something that you do for yourself’.
A dissertation also “gives evidence of the candidate’s mastery […] of scholarly method”. This sounds terribly daunting but don’t be put off. The phrase is telling you that you will have to lift your game to write a successful dissertation. ‘Scholarly method’ means that you will be expected to do more and better reading and research than for a standard undergraduate essay. It means that your work will display accuracy and skill in its investigation and discussion of a subject. It means that your discussion will give evidence of critical analysis i.e. standing back from your subject and weighing up pros and cons. It means you will show that you understand that, for example, aspects of particular theories or viewpoints are open to question.