The definition "A collective behavior exhibited by organsims which group together." (Source: Softchalk Lesson) Animals in a swarm don't need to physically communicate to know how to move, where to move and what to do, if you ever see a video or a physical swarm itself, they seem to flow together flawlessly as if they were conducted to do so. I read that in a swarm of birds, each bird tracks at least seven other birds movements and follows them, and thats what keeps them from crashing into each other. 

This behavior is usually exhibited by animals that are prey and and are kind of small in comparison to the other organisms. The swarm behavior was adapted so that the individual organisms could work together to try and trick the predators or to intimidate them. They also may move as a group so they can catch their food more easily or so that they can gain warmth and shelter from each others bodies.