Definition: A denial of service attack is one of many threat vectors that is arrayed against an information infrastructure. A denial of service attack is an overwhelming number of requests being sent to the targeted computer or device. So many requests for service are sent that the device is overwhelmed and unable to respond in a timely manner. Thus the function the computer or server is supposed to perform is nullified. Physical harm to systems and people can be the result.
Its Relevance: Catastrophic damage can occur if an otherwise properly functioning network infrastructure if vital information is made to be unavailable to those who need it for decision making. For example, imagine a pressure valve that is controlled via the Internet. If the control value is beyond being reached an explosion can occur.