A DistributedDenialOfService (DDoS) attack is a DenialOfService attack originating from multiple computers acting in concert. The computers can be acting either wittingly, or unwittingly. There have been many instances of crackers compromising a system and using it in a DDoS attack against another system.
Typically done to increase the "bandwidth" available for a DenialOfService attack. A cracker who's got a 768K DSL line isn't going to be able to take down Google. Several thousand computers, each with 768K DSL lines can likely deny service. Given a large enough number of nodes in a DDoS attack, any site can be brought to its knees.
Also done to disguise the origins of an attack.