The US government accused Iran of conducting a prolonged series of DDoS against the web sites of Bank of America and other financial institutions, presumably as retaliation for economic sanctions levied against Iran for its nuclear program. Recently DDoS attacks by extortionists have targeted banks in Greece and Sweden. So what are DoS and DDoS attacks?

DoS stands for "denial of service" and refers to an attack that overwhelms a system with data—most commonly a flood of simultaneous requests sent to a website to view its pages, causing the web server to crash or simply become inoperable as it struggles to respond to more requests than it can handle. As a result, legitimate users who try to access the web site controlled by the server are unable to do so. There are other types of DoS attacks that use different tactics, but they all have the same effect: preventing legitimate users from accessing a system or site.