A dissertation is a formal document prepared for the purpose of attaining a doctorate degree at a university. The dissertation is usually a very lengthy document prepared through literature review, and the use of primary and secondary sources of information. Extensive research in terms of experiments, surveys and interviews are usually carried out to form the primary research. While secondary research is conducted by analysing already existing works of other scholars which are relevant to the topic of the dissertation.
A dissertation has the main aim of reporting an issue or a problem, identifying the source and providing the solution or recommendations through research for solving the identified problem.
The essay on the other hand is a shorter document which is composed around one subject or topic and usually presents the personal view of the author. The scope of the essay is restricted to the topic at hand only and the author as a result attempts to use substantial research relevant to the topic to justify his point of view while simultaneously persuading the reader.
In terms of the academia, an essay is a regular research report pr a piece if literature which is written to describe and analyse a topic form a point of view while the dissertation is a formal document which aims to identify a problem, find its sources, analyse it and provide recommendations for solving the problem or issue at hand.