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Sociology is the scientific study of society.[1] It is a social science (a term with which it is sometimes synonymous) which uses various methods of empirical investigation[2] and critical analysis[3] to develop a body of knowledge about human social activity. For many sociologists the goal is to conduct research which may be applied directly to social policy and welfare, while others focus primarily on refining the theoretical understanding of social processes. Subject matter ranges from the micro level of individual agency and interaction to the macro level of systems and the social structure.[4]

The traditional focuses of sociology have included social stratification, social class, culture, social mobility, religion, secularisation, law, and deviance. As all spheres of human activity are affected by the interplay between social structure and individual agency, sociology has gradually expanded its focus to further subjects, such as health, medical, military and penal institutions, the Internet, and the role of social activity in the development of scientific knowledge.

http://en.wikipedia.org/wiki/Sociology

Specifically, it is the study of group behavior and human interaction.  It teaches us why, for example, people in groups often behave more poorly than individuals do, and how group decision-making is often better than that made by a single individual.