A denial of service (DOS/DoS) attack prevents a machine from carrying out its main duties - for example, preventing a WebServer from serving up pages.
A distributed denial of service (DDoS) attack normally means many clients are sending repeated requests to a machine to saturate its bandwidth, preventing legitimate requests to get through. These clients have typically been infected with a Virus or Worm and are sent instructions regarding when and where to attack. They are then known as "zombies".
6 pages link to DenialOfService:
