Many living beings such as birds, fish, insects, and land animals perform specific operations such as moving, hunting, and foraging in groups. They stay and hunt in groups because it makes them stronger and safer from predators than pursuing goals individually. So, let's say you want a group of birds flocking, swarming around in the sky; it'll cost too much time and effort for animators to design the movement and animations of each bird. However, if we apply some simple rules for each bird to follow, we can achieve the emergent intelligence of the whole group with complex, global behavior.
One pioneer of this concept is Craig Reynolds, who presented such a flocking algorithm in his 1987 SIGGRAPH paper, Flocks, ...
