In the narrowest sense, comparison shows how two or more things are similar, and contrast shows how they are different. In most writing situations, however, you use the two related processes of comparison and contrast to consider both similarities and differences. In the following paragraph from Disturbing the Universe, Freeman Dyson compares and contrasts two different styles of human endeavor, which he calls "the gray and the green."

In everything we undertake, either on earth or in the sky, we have a choice of two styles, which I call the gray and the green. 'I'he distinction between the gray and the green is not sharp. Only at the extremes of the spectrum can we say without qualification, this is green and that is gray. The difference between green and gray is better explained by examples than by definitions. Factories are gray, gardens are green. Physics is gray, biology is green. Plutonium is gray, horse manure is green. Bureaucracy is gray, pioneer communities are green. Self-reproducing machines are gray, trees and children are green. Human technology is gray, God's technology is green. Clones are gray, clades* are green. Army field manuals are gray, poems are green.

                                                                      * Clades are groups of organisms that evolved from a common ancestor.

A special form of comparison, called analogy, looks for similarities between two essentially dissimilar things. With analogy you explain one thing by comparing it to a second thing that is more familiar than the first. In the following paragraph from The Shopping Mall High School, Arthur G. Powell, Eleanor Farrar, and David K. Cohen use analogy to shed light on the nature of American high schools.

            If Americans want to understand their high schools at work, they should imagine them as shopping malls.
           Secondary education is another consumption experience in an abundant society. Shopping malls attract a
            broad range of customers with different tastes and purposes. Some shop at Sears, others at Woolworth's
            or Bloomingdale's. In high schools a broad range of students also shop. They too can select from an
            astonishing variety of products and services conveniently assembled in one place with ample parking.
            Furthermore, in malls and schools many different kinds of transactions are possible. Both institutions
            bring hopeful purveyors and potential purchasers together. The former hope to maximize sales but can
            take nothing for granted. Shoppers have a wide discretion not only about what to buy but also about
            whether to buy.

Throughout our lives we are bombarded with countless bits of information from newspapers, television, radio, and personal experience: The police strike in Memphis; city workers walk out in Philadelphia; the Senate debates government spending; taxes are lowered in New Jersey. Somehow we must make sense of the jumbled facts and figures that surround us. One way we have of understanding information like this is to put it side by side with other data and then to compare and contrast. Do the police in Memphis have the same complaints as the city workers in Philadelphia? What are the differences between the two situations? Is the national de- bate on spending analogous to the New Jersey debate on taxes? How do they differ? We make similar distinctions every day about matters that directly affect us. When we make personal decisions, we consider alternatives, asking ourselves whether one option seems better than another. Should I buy a car with manual or automatic transmission? Should I major in history or business? What job opportunities will each major offer me? Should I register as a Democrat or a Republican, or should I join a smaller political party? What are the positions of each on government spending, welfare, and taxes?
Because this way of thinking is central to our understanding of the, world, comparison and contrast is often called for in papers and on essay examinations:

Compare and contrast the attitudes toward science and technology expressed in Fritz Lang's Metropolis and George Lucas's Star Wars. (film)

What are the similarities and differences between mitosis and meiosis? (biology)

Discuss the relative merits of establishing a partnership and a corporation. (business law)

Discuss the advantages and disadvantages of heterogeneous grouping of pupils. (education)

You are not likely to sit down and say to yourself, "I think I'll write a comparison-and-contrast essay today. Now what can I write about?" Instead, you will use comparison and contrast because your assignment suggests that you do so or because you decide it suits your purpose. In the examples above, for instance, the instructors have phrased their questions to tell students how to treat the material. When you read the questions, certain key words and phrases--compare and contrast, similarities and differences, relative merits, advantages and disadvantages--indicate that you should use a comparison-and-contrast pattern to organize your essay. (Sometimes you may not even need such key phrases. Consider the question, "Which of the two Adamses,  John or Samuel, had the greater influence on the timing and course of the American Revolution?" The word greater is enough to suggest a contrast.
 Even when you are not given an assignment that is worded to suggest comparison and contrast, your purpose may point to this organization strategy. For instance, when you evaluate, you frequently employ comparison and contrast. If, as a student in a course in hospital management, you were asked to evaluate two healthcare systems, you could begin by researching the standards used by experts in their evaluations. You could then compare each system's performance with those standards, and then contrast the systems with each other, concluding perhaps that both systems met minimum standards but that one was more cost-efficient than the other. Or if you were evaluating two of this year's new cars for a consumer newsletter, you might establish some criteria--fuel economy, handling, comfort, sturdiness, style--and compare and contrast the cars with respect to each criterion. If each of the cars was better in different categories, your readers would have to decide which features mattered most.

 When you know what subjects you will compare and contrast, you then need to select the points you want to discuss. You do this by determining your emphasis--on similarities, differences, or both--and the major focus of your paper. If your purpose for comparing two types of house plants is to explain that one is easier to grow than the other, you would contrast points having to do with plant care, not those having to do with plant biology. When you compare and contrast, make sure that you treat the same, or at least similar, elements for each subject you discuss. For instance, if you were going to compare and contrast two novels, you might consider the following elements in both works:

                                                          Novel A                                                                                   Novel B
                                                   Major characters                                                                       Major characters
                                                   Minor characters                                                                       Minor characters
                                                   Themes                                                                                     Themes
 

You should avoid the common error of discussing entirely different elements for each subject. Such an approach obscures any basis of comparison that might exist. The two novels, for example, could not be meaningfully compared or contrasted if you discussed elements such as these:
                                                     Novel A                                                                                          Novel B
                                                 Major characters                                                                             Plot
                                                 Minor characters                                                                             Author's life
                                                 Themes                                                                                           Symbolism

 After you decide on the points you want to discuss, you are ready to formulate your thesis statement. This thesis establishes the significance of the comparison or contrast and perhaps the relative merits of the items discussed.
 As in other kinds of essays, your thesis statement should tell your readers what to expect in your essay. It should mention not only the subjects to be compared and contrasted but also the point you will make about them. Your thesis should also indicate whether you will concentrate on similarities or differences or whether you will balance the two. In addition, it may list the points of comparison and contrast in the order in which they are discussed in the essay.
 The structure of your thesis sentence can help to show the focus of your essay. As the following sentences illustrate, a thesis statement can emphasize the central concern of the essay by stating it in the main, rather than the subordinate, clause of the sentence. The structure of the first sentence emphasizes similarities, and the structure of the second highlights differences:

Despite the fact that doctors and nurses perform distinctly different tasks at a hospital, their functions overlap when they interact with patients.

Although Melville's Moby-Dick and London's The Sea Wolf are both about the sea, the major characters, minor characters, and themes of Moby-Dick establish its greater complexity.

 Like every other type of essay examined in this book, a comparison-and-contrast essay has an introduction, several body paragraphs, and a conclusion. Within the body of your paper, there are two basic comparison-and-contrast patterns you can follow: You can discuss each subject separately, devoting one or more paragraphs to subject A and then the same number to subject B; or you can discuss one element common to the two subjects in each section of the paper, making your points about subject A and subject B in turn. As you might expect, both organizational patterns have advantages and disadvantages that you should consider before you use them.

Using Subject-by-Subject Comparison
When composing a subject-by-subject comparison, you essentially write a separate essay about each subject, but you discuss the same points for both subjects. In discussing each subject, you use the same basis of comparison to guide your selection of supporting points, and you arrange these points in some logical order, usually in order of their increasing significance. The following informal outline illustrates a subject-by-subject comparison:

Introduction:   Thesis statement--Despite the fact that doctors and nurses perform distinctly different tasks at a hospital, their functions overlap when they interact with patients.

Doctors' functions
Point 1:        Teaching patients
Point 2:        Assessing patients
Point 3:        Dispensing medication

Nurses' functions
Point 1:        Teaching patients
Point 2:        Assessing patients
Point 3:        Dispensing medication

Conclusion:     Reiteration of thesis

Subject-by-subject comparisons are most appropriate for short, uncomplicated papers. In longer papers, where many points are made about each subject, this organizational pattern puts too many demands on your readers, requiring them to keep track of all your points throughout your paper. In addition, because of the length of each section, your paper may seem less like a unified, coherent whole than like two separate essays weakly connected by a transitional phrase.  For longer or more complex papers, then, it is best to use a point-by-point comparison.

Using Point-by-Point Comparison

When you write a point-by-point comparison, you first make a point about one subject and then follow it with a comparable point about the other. This alternating pattern continues throughout the body of-your essay until all your comparisons or contrasts have been made. The following informal outline illustrates a point-by-point comparison:

Introduction: Thesis statement-Melville's Moby-Dick has more fully developed characters and more complex themes than does London's The Sea Wolf

Minor characters
Book 1:      The Sea Wolf
Book 2:      Moby-Dick

Major characters
Book 1:      The Sea Wolf
Book 2:      Moby-Dick

Themes
Book 1:      The Sea Wolf
Book 2:      Moby-Dick
Conclusion:  Restatement of thesis

Point-by-point comparisons are especially useful for longer, more complicated essays where you discuss a number of different points. (If you treat only one or two points of comparison, then you should consider a subject-by-subject organization.) In the point-by-point pattern, readers can easily follow comparisons or contrasts and do not have to wait several paragraphs to find out the differences between Moby-Dick and The Sea Wolf or to remember on page six what was said on page three. Nevertheless, it is easy to fall into a monotonous, back-and-forth movement between points when you write a point-by-point comparison. To avoid this problem, use clear transitions, and vary sentence structure as you move from point to point.

Taken from Patterns for College Writing. 6th Ed. Ed. Laurie G. Kirszner and Stephen R. Mandell. New York: St. Martin's, 1995. 353-359.