To compare two or more items means to find the similarities, or ways in which the items are alike, and to contrast them means to find how they are different. Teachers, no matter what the subject, frequently ask students to compare and contrast items or ideas to encourage them to examine individual concepts thoroughly.
The ability to compare and contrast requires a higher level of thinking than simple knowledge about a topic. Bloom's Taxonomy of the Cognitive Domain lists the various levels of thinking and learning in ascending order of difficulty. Comparing and contrasting falls in the category of analysis, which is the fourth level.
Many English teachers specifically teach how to write a compare-contrast essay, because the ability to compare and contrast and to express how ideas are alike and different are common expectations the higher one goes in education. Instructors often suggest the use of a Venn diagram for preparing to write such an essay. A Venn diagram uses overlapping circles, with one circle per item examined. Students list traits unique to a particular item in the part of the circle that does not overlap and shared traits in the overlapping area.
Compare and contrast exercises often occur in literature classes, finding similarities and differences between poems, novels or authors. History courses call for comparing and contrasting between groups, leaders and historical times. In science, students compare and contrast certain lab results, life forms and elements.