LOIC ("Low Orbit Ion Cannon") is an application developed by 4Chan-affiliated hackers designed to--when used en masse by thousands of anonymous users--launch Distributed Denial of Service (DDoS) attacks on websites. Like Visa.com and Mastercard.com, for instance.It's interesting to see the tweets from Operation Payback on Twitter. It looks like they've also gone after Joe Leiberman's site, as well. I'm sure he'll have some choice things to say about it. No doubt he is loving every second of the attention he's garnering from all this.
It's a pushbutton application...
The idea behind LOIC is that it can allow you to participate in attacks even if you've no clue how to hack. Just download a copy of LOIC (available for Windows, Mac, and Linux!), punch in the target information like a URL or an IP address and zap.
UPDATE: Operation Payback just published a huge list of credit card numbers with expiration dates. (I saw the list, but I'm not going to link to it.) I don't know if they just grabbed these numbers today, or if they are real credit card numbers. Things are going to get worse before they get better.
UPDATE 3:31PM est: The leaked MasterCard numbers might have been faked. (Can I have a T-shirt with that logo?)