The roster of websites that have already signed up reads like the Internet's own version of the Justice League. According to AllThingsD, "WordPress, Imgur, Reddit, Cheezburger Network, Public Knowledge, the Electronic Frontier Foundation and Craigslist founder Craig Newmark" have all agreed to participate.
The group plans to help organize websites around a specific issue that might threaten the Internet as we know it. Whenever the Defense League wants to organize a new cause, they send out code to participating websites that adds a pop up message -- the 'cat signal' -- for every new user that visits the site. The message will ask visitors to participate in some form of protest, like "a prominent message asking everyone to call their elected leaders," according to the IDL's website.
As the IDL point out on their website, the mass protests and blackouts helped beat SOPA and PIPA in the past, and the aim of the new group is to be able to organize protests and blackouts more quickly should another bill threaten the way the Internet operates.