Throughout an academic career, a student may be asked to write many essays on many different topics. Only some students complete their studies in a university's doctoral program, and only those students may be asked to write a dissertation.
The purpose of an essay is to inform others on a particular topic. An essay writer offers their thoughts on a subject and provides some supporting arguments. The way that the writer presents their information will fall within the four traditional categories of an essay: persuasive, expository, descriptive, or narrative. Writers can decide if the information they want to present is best expressed by telling a personal story or through simply recounting the facts around the topic.
A dissertation is meant to add something new to a particular field of study. Through careful research, as well as the application of all the knowledge a dissertation writer has learned through many years of study in their field, a dissertation is written to introduce an entirely new thought to an existing body of knowledge in that field of study.
Dissertations are written to be published in academic journals to disseminate these new ideas among scholars and help to guide future study on the chosen topic.
The audience for an essay is, most often, the teacher or professor who assigned the essay. Some essays are published in magazines and in newspapers, and some end up in compilations of essays on a particular topic.
The audience for a dissertation is an academic committee and every other scholar in the writer's chosen field. A committee will accept or reject a dissertation based on many factors, such as whether the writer has sufficiently proven their point and provided a significant amount of supporting arguments as well as a comprehensive study of prior written works on the subject.
The biggest differences between an essay and a dissertation are the requirements involved. An essay can be considered complete, depending on the type of essay, when only presenting outside information or personal experiences on a topic. Some essays do not require a writer to form their own opinion or present an analysis at all, such as an expository essay.
A dissertation is not sufficient to complete a doctoral program if the writer has not been able to critically analyze their topic in a new way. Simply presenting prior research or existing facts can take up many pages of writing, but it won't be enough for a dissertation if the writer hasn't expressed their own, original thoughts.