If you've never heard of Digg, it's a popular social bookmarking/user submitted news site. Users submit links and other users can 'Digg it' (+1 point) if they liked it and bury it (-1 point) if they don't. The front page of Digg.com lists the hottest upcoming stories at that moment. As a story gets more Diggs, it is more prominently featured, and you get a snowball effect with more and more users flocking to that story.