This document discusses the definition and classification of groups according to sociologist Richard Schaefer. It defines a group as a number of people who regularly interact and share common norms and values. Groups are distinguished from crowds based on the strength of interaction and attachment between members. Schaefer classified groups into four main types: primary groups which are small and intimate, secondary groups which are larger and more impersonal, in-groups which members feel a strong sense of belonging, and out-groups which members feel antagonism towards. The document also discusses the concepts of reference groups which influence members' behaviors and norms, and networks which describe the connections between individuals, groups, and organizations at micro, meso, and macro levels.