There's a reason you won't be asked to write multiple theses for one class or even in one academic year. While you definitely need to have some knowledge on a topic to write an essay, writing a thesis requires you to become an expert on your topic. In other words, a thesis is a lot of work.
An essay should include an introduction that includes a thesis statement, which is one sentence that summarizes the writer's purpose or point, just as a full thesis does. An essay will then traditionally include three or more body paragraphs with topic sentences that elaborate on the point and a conclusion paragraph that explains how the points made in the body of the essay prove the writer's thesis statement.
Most essays can and do include outside arguments from written pieces of work to help support the writer's conclusions. A short bibliography, or works cited page, should be included at the end of an essay that refers to or quotes other written works.
A thesis traditionally includes an abstract as well as an entire section of several paragraphs that introduce the writer's topic and the outside works a writer will be analyzing to reach their conclusions. The introductory section is followed by a body of writing that examines prior written works on the topic and proposes how to look at the topic from a new angle. A final section should both summarize the points made and explain how the thesis writer synthesized those prior works to form the new and original conclusion the thesis writer has reached on that topic.
An average essay is anywhere from one to five pages long. Five to seven paragraphs, including the introduction and conclusion paragraphs, are sufficient for an essay, although some are longer.
The average thesis for an undergraduate writer is 40 to 60 pages long and an average graduate or doctoral thesis is between 60 to 100 pages.
The point of an essay can be to tell a story, describe an event or other topic, persuade a reader to agree with a certain viewpoint, or relate information in a strictly factual way.
The point of a thesis is to look at information in a new way and present a comprehensive look at how the topic has been analyzed before. A thesis uses the knowledge learned from an entire college program to create an expert opinion that can be fully explained and supported.