Anonymous called the attack “Operation Payback”. This was in response to the shut down of file-sharing site Megaupload by the FBI after a DOJ indictment was passed on the company in a Virginia district court.
The websites were taken down by a DDoS (Distributed Denial of Service) attack. A DDoS attacks works by flooding a website with a huge number of requests simultaneously and repeatedly. Around 6,000 people took part in the attack. Some reports however indicate that a number of people who supposedly took part were unwittingly involved in the attack.
Anonymous has increasingly turned towards the Low Orbit Ion Cannon (LOIC) program to carry out its cyber attacks. Anonymous has made it easy for the average computer user to take part in such attacks by making LOIC easily available. This includes posting links to the software on popular social networking sites and a site where the unsuspecting visitor’s computer becomes part of the PC army carrying out the DDoS attack.