Distributed Denial of Service (DDoS)

A distributed denial of service (DDoS) attack is a denial of service (DoS) attack launched from multiple simultaneous hosts.

DDOS attacks are often launched from botnets. The attacks flow directly from the compromised hosts included in the botnet. These hosts, under botnet control, are called zombies.

References:

http://www.cert.org/homeusers/ddos.html

http://staff.washington.edu/dittrich/misc/ddos/