Group Behaviour refers to people with similar goals behaving in the same way. Characteristics of groups include having 2 or more persons, a formal social structure, common goals, face-to-face interaction, and interdependence. There are several types of groups such as informal groups, formal groups, task groups, command groups, and interest groups. People join groups for reasons like belonging, feeling superior, survival, identity projection, and self-esteem. Groups go through five stages of development and have properties like norms, cohesiveness, roles, status, and size that influence group behavior.