Operation Payback is a coordinated,[1decentralized[2] group of attacks on opponents of internet piracy by internet activists using the "Anonymous" moniker. Operation Payback started as retaliation to distributed denial of service (DDoS) attacks on torrent sites; piracy proponents then decided to launch DDoS attacks on piracy opponents. The initial reaction snowballed into a wave of attacks on major pro-copyright and anti-piracy organizations, law firms, and individuals. Following the United States diplomatic cables leak in December 2010, the organizers commenced DDoS attacks on websites of banks who had withdrawn banking facilities from WikiLeaks.
Source:
http://en.wikipedia.org/wiki/Operation_Payback