From Wikipedia:

Social psychology is the scientific study of how people’s thoughtsfeelings, and behaviors are influenced by the actual, imagined, or implied presence of others.[1] In this definition, scientific refers to the empirical investigation using the scientific method. The terms thoughtsfeelings, and behavior refer to psychological variables that can be measured in humans. The statement that others’ presence may be imagined or implied suggests that humans are malleable to social influences even when alone, such as when watching television or following internalized cultural norms. Social psychologists typically explain human behavior as a result of the interaction of mental states and social situations.

Social psychologists examine factors that cause behaviors to unfold in a given way in the presence of others. They study conditions under which certain behavior, actions, and feelings occur. Social psychology is concerned with the way these feelings, thoughts, beliefs, intentions, and goals are cognitively constructed and how these mental representations, in turn, influence our interactions with others.

Social psychology traditionally bridged the gap between psychology and sociology. During the years immediately following World War II there was frequent collaboration between psychologists and sociologists.[2] The two disciplines, however, have become increasingly specialized and isolated from each other in recent years, with sociologists focusing on “macro variables” (e.g., social structure) to a much greater extent than psychologists.[citation needed] Nevertheless, sociological approaches to psychology remain an important counterpart to psychological research in this area.

In addition to the split between psychology and sociology, there has been a somewhat less pronounced difference in emphasis between American social psychologists and European social psychologists. As a generalization, American researchers traditionally have focused more on the individual, whereas Europeans have paid more attention to group level phenomena (see group dynamics).[3]