In a classification essay, a writer organizes, or sorts, things into categories.
The classification essay organizes supporting details into three or more groups with each group having its own identifying characteristics. For this reason, classification essays rely heavily on description and example as supporting details, description providing the means for identifying distinguishing characteristics and examples illustrating the members of each class.

There are three elements to a classification essay: the set of things being classified, the scheme (or principle of selection) for classifying the set, and the classes into which the set is divided.

Set: Any set, which is to be classified, must share a common characteristic or be able to be distinguished from things outside the set. Biology classifies living organisms (non-living materials lay outside the set). Students, pets, vacation spots, or other generic nouns can serve as a set to be classified.

Scheme: A scheme is the quality, standard, type, or function by which the elements of the set are sorted into classes, that is, the principle of selection for creating the groups. Biologists classify living organisms based on life processes and qualities. Geologists sort rocks into igneous, sedimentary, and metamorphic rocks, a classification based on how the rocks were formed. In order for a classification scheme to be effective, it must be a significant, important, relevant, or valuable way to understand the set of things under consideration.

Classes: In general, when writing about each class, begin with an explanation of the descriptor, the characteristic which identifies the members of one class and differentiates the members of that class from the members of another class. Then include representative examples of the class. Often, a discussion of the class also includes an evaluation of the worth of this class or a comparison/contrast of this class to other classes within the scheme. Biologists frequently refer to the ways in which members of a class are differentiated from members of other classes. For these reasons, modes, which are used as methods within classification often, include description, narration, definition, example, and comparison/contrast.