From the website:

Digg is a user driven social content website. Ok, so what the heck does that mean? Well, everything on Digg is submitted by our community (that would be you). After you submit content, other people read your submission and Digg what they like best. If your story rocks and receives enough Diggs, it is promoted to the front page for the millions of visitors to see.

Digg gets 17 million unique visitors a month.