The ability to break a concept into its similar and dissimilar characteristics allows students to understand (and often solve) complex problems by analyzing them in a more simple way. Teachers can either directly present similarities and differences, accompanied by deep discussion and inquiry, or simply ask students to identify similarities and differences on their own. While teacher-directed activities focus on identifying specific items, student directed activities encourage variation and broaden understanding, research shows. Research also notes that graphic forms are a good way to represent similarities and differences.
(From Marzano's Nine Instructional Strategies for Effective Teaching and Learning, http://www.ntuaft.com/TISE/Research-Based%20Instructional%20Strategies/marzanos%209%20strategies.pdf).