This started out as a simple caption competition, with an artist posting a cartoon and captioning it, and then challenging the general public to do better. Over time, it's grown into something more, as witnessed by the section where you can view details of some of the more prolific captioners.
The quality of caption varies, of course, but on the whole they're pretty entertaining. What I really enjoyed about visiting this site was the feeling of community, the message boards showing how it's not just a set of annonymous people posting their takes on the picture of the day.
Interesting site that deserves more attention that it gets.