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).
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.