A Denial-of-service attack (DoS) occurs when someone directs a large number of electronic requests to a target URL; the requests occur so quickly that the Web server can't respond and the site becomes inaccessible to everyone.
A distributed denial-of-service attack (DDoS) occurs when hundreds or thousands of compromised computers are enlisted. Such clusters of compromised computers is typically referred to as a BotNet. Many said BotNets are usually clustered in central mIRC channels.
- Cyber war in Estonia from C|Net (5.21.07)