This document summarizes key concepts around groups and group behavior from Organizational Behavior by Stephen P. Robbins. It defines groups and describes different types of groups. It then covers reasons people join groups, models of group development, group roles and norms, and factors that influence group structure, tasks, decision-making, and performance. Specifically, it discusses Tuckman's five stages of group development, norms, social loafing, and techniques for group decision-making.