Answer:
Social Psychology is a particular branch of Psychology - I would not say that it "differs" from Psychology, but rather that it is a specialization or type of Psychology. It focuses not only on groups of people, but also on how individuals interact with other people whether in large or small groups. Some particular areas of interest within social psychology are persuasion, in-groups and out-groups, decision-making, and stereotyping. So although Social Psych. is a sub-field of Psychology, it is still quite broad and varied.