Swarm is a graphical map of hundreds of websites, all connecting to each other. It updates itself every second with where people are going and coming from. As sites become more popular, they move towards the center of the swarm.
Website traffic is symbolized with thin lines. Each time you see a line appear, it means someone has moved from one site to the other. You can gauge how many people are swarming around based on the number of lines.
In addition to a forum, Swarm allows you to chat about any link you see. When you see a little chat icon flash, it means people are actively chatting about that link.There's a Firefox extension, that anonymously submits the websites you visit, making you an integrated part of the community, and there's even a simplified text version of Swarm that doesn't require Adobe Flash.