In nature, we can observe what we refer to as flocking behavior in several species. Flocking simply refers to a group moving in unison. Schools of fish, flocks of sheep, and cicada swarms are fantastic examples of this behavior. Modeling this behavior using manual means, such as animation, can be very time-consuming and is not very dynamic. In Chapter 5, Flocks and Crowds, we'll explore a dynamic and programmatic approach to modeling this behavior in a believable way, using a simple set of rules that will drive the behavior of the group and each individual in a group relative to its surroundings.
Similarly, crowds of humans, be it on foot or in vehicles, can be modeled by representing the entire crowd as an entity rather than trying to model each individual as its own agent. Each individual in the group only really needs to know where the group is heading and what their nearest neighbor is up to in order to function as part of the system.