Computational Sociology and Agent-Based Modeling
Consider a flock of geese flying in tight formation. Collectively, they form the image of a giant delta-shaped bird that moves as purposively as if it were a single organism. Yet the flock has no "group mind" nor is there a "leader bird" choreographing the formation (Resnick 1994). Rather, each bird reacts to the movement of its immediate neighbors who in turn react to it. The result is the graceful dance-like movement of the flock whose hypnotic rhythm is clearly patterned yet also highly non-linear.
Read the article by Michael W. Macy & Robert Willer