Exposition is one of the four chief types of composition. The others include argumentation, description, and narration. In high school as well as English 101 and 102 of your freshman year in college, you can expect to write papers which fall into each of these categories.
The purpose of exposition is to explain something, and this may be done through several methods. A few of the methods include identification, definition, classification, comparison and analysis.
In drama, the exposition is the introductory material which creates the tone and mood of the piece, creates the setting, introduces the characters, and supplies other facts the reader/audience needs in order to understand the development of the plot and character motives.