A "denial-of-service" attack is characterized by an explicit attempt by attackers to prevent legitimate users of a service from using that service. Examples include



** attempts to "flood" a network, thereby preventing legitimate network traffic

** attempts to disrupt connections between two machines, thereby preventing access to a service

** attempts to prevent a particular individual from accessing a service

** attempts to disrupt service to a specific system or person 



Not all service outages, even those that result from malicious activity, are necessarily denial-of-service attacks. Other types of attack may include a denial of service as a component, but the denial of service may be part of a larger attack.



Thanks to CERT.



*(http://www.cert.org/tech_tips/denial_of_service.html CERT's full Dos summary)