Exposition is an explanation of a theory, plan. Half of our speech is exposition, though we may not be aware of that fact. It plays an important part in our everyday relationships. The office equipment salesman explains to his prospective buyer the superiority of steel desks over those made of wood; the physician explains to his patient the dangers of infection; the mother explains to her child what makes the day and night.
Exposition has as its object making clear a general term or principle. In this it differs from description and narration, both of which describe specific instances of things. If you embody description and narration in your explanation, you must subordinate them.
There are few simple rules that everyone can follow to achieve the essential clearness of the exposition:
1) know your subject matter thoroughly;
2) present your statements in logical order; (start with the simple and proceed to the complex);
3) be as brief as is consistent with an adequate explanation;
4) suit your language to your audience;
5) illustrate general principles by concrete examples; (introduce comparison, contrast, description, narration);
6) use precise terms;
7) make exact statements.
Perhaps, the most familiar kind of formal exposition is the newspaper or magazine editorial, because they use some event of immediate interest as an example of a general truth.
Other forms of exposition are the essay, the monograph and the treatise (textbooks and other long didactic explanations).