"Expository essay" is actually a category for other essay types. All expository essay types have thesis statements, introductions, conclusions, arguments and supporting facts. These types are defined by Dr. A. Ball of Stanford University as "expository organizational patterns." These organizational patterns include: description (or, descriptive essay); comparison (or, comparison essay); sequence; cause and effect; and problem and solution. Two other organizational patterns that may be included are classification and definition; process analysis may also be included but it is quite similar to the sequence organizational pattern.
Two essay types not included as expository, even though they too require thesis statements, argument and supporting facts, are persuasive and discursive. "Why?" you ask. The reason is that these are opinion-driven, whereas expository essays are fact-drive, excluding opinion altogether.
Knowing this prompts the question, "What is an expository essay?" Expository essays are ones that explain, describe, provide information or inform (slightly different from providing information) (Ball). Expository essays assume no prior information on the topic is known by the reader. Exposition uses clear, descriptive or informative vocabulary rather than "blatant" vocabulary: the reader is lead to clearly understand the points of the essay. [Dr. Ball, formerly of University of Michigan, provides graphical organizers for most expository organizational patterns.]
Writing an expository essay in any organizational pattern requires perhaps less research than other essay types, though you must certainly be well acquainted with all the particulars of your topic, generally necessitating research to some extent. Your research and own understanding will form the basis of your essay argument proving the thesis statement presented in your Introduction. To give an illustration of how an argument is an integral part of expository writing, consider a comparison between a songbird and an ocean.
While on the surface, such a comparison seems unlikely, suppose I give this as a thesis statement: "Though unlike in most aspects, a songbird and ocean are alike in the effects they produce." I would then prove this by arguing similar effects. Both produce soothing sounds: the one, a lilting song; the other, a soothing roar of waves. Both produce a pleasing breeze: the one, a stirring flutter of wings in the air; the other, the refreshing landward wind. Researched facts will provide my support evidence and, by my Conclusion, I will have proven my thesis.
It may be harder to conceive of a thesis and argument in relation to a descriptive expository essay, it nonetheless is applicable. To illustrate, suppose I want to describe the air in South Africa. My thesis may pose this assertion: "Though all air has the same chemical composition, air is nonetheless affected by the mineral content of soil and water in local vicinities rendering distinct differences in air characteristics, as is true for South African air." To prove my assertion, I will go on to describe South African air providing mineral data as argument proofs.
Expository introductions and conclusions, which begin and end your essay, respectively, serve as in other essays to (1) orient attention to the topic and pose your assertion in a thesis, then to (2) reiterate that thesis with comment on how you've proven it while offering suggestion of a continuing significance to your topic, perhaps suggesting future measurements of wing movement or trace elements in air.
The expository essay sets forth information and explanation on the subject using facts, opinions and details. There are actually three basic ways of writing the expository essay. First is the recognition of a pattern with the use of models. Second the process or art of selecting and collecting appropriate information which may include making a skeleton or writing an outline. The last one is production, the state when the writer fleshes out the skeleton.