Flocking is a technique for animating groups of creatures, such as flocks of birds, schools of fish, herds of wildebeest, etc.
It is described as a type of artificial life algorithm, or emergent behavior. This means that it uses a set of simple rules to control digital characters, and that the rules lead to more complex behaviors that mimic living things.
Flocking was introduced in 1987 by Craig Reynolds, and used in the classic computer animation "Stanley and Stella: Breaking the Ice". http://www.red3d.com/cwr/boids/ is Reynolds' official web page on flocking.
The generic term boid is often used to refer to artificial creatures that use flocking behavior.