Refers to DoS (Denial of Service) - an acronym for the disruption that results from an attack on a network. Hackers execute a DoS with the intent of blocking individuals' ability to access Web sites or e-mail. The most common form of DoS results from an attacker who sends so many messages to a Web site that it can't respond to regular users or it shuts down completely (see: broadcast storm). DoS is a criminal act, even if done as a prank; major attacks are aggressively investigated by the U.S. Department of Justice.
DDoS stands for "Distributed Denial of Service," an attack in which a multitude of compromised systems attack a single target, thereby causing a DoS for users of the targeted system. The flood of incoming messages to the target system essentially forces it to shut down, thereby denying service to legitimate users.
Historical perspective: Banking: Cyberattacks frustrate bank customers - In October 2012, six major U.S. banks were hit with day-long cyberattacks according to The New York Times. A hacker group with Middle Eastern ties claimed responsibility for the unsophisticated but effective denial-of-service attacks, which it said were in retaliation for an anti-Islam video that mocks the Prophet Mohammed. Accounts at Bank of America, JPMorgan Chase, Citigroup, U.S. Bank, Wells Fargo, and PNC were reportedly not compromised, though customers could not access their money or pay bills online.