it is a website where users submit links to stories, videos or podcasts that they find interesting and then other users "digg" the submitted stories. The stories are ranked according to how many diggs they get. It's an interesting site, and if you haven't been there you really should check it out.
My favorite part of Digg is the Digg Labs where you can view the stories as Swarms or Stacks. They are both visual representations of stories being submitted and dugg in real time. In Swarm, stories are represented as circles. You can see Digg users swarming around the stories and watch them grow as their popularity increases. In the Stacks view stories are shown as columns and diggs fall from above. The columns change color to represent the number of diggs they have received.