What is DDoS attack ?

What is DDoS attack ?

in reference to:

"Short for Distributed Denial of Service, it
is an attack where multiple compromised systems (which are usually infected with
a Trojan) are used to target a single system causing a
Denial of Service (DoS) attack. Victims of a DDoS attack consist of both the
end targeted system and all systems maliciously used and controlled by the
hacker in the distributed attack."
- What is DDoS attack? - A Word Definition From the Webopedia Computer Dictionary (view on Google Sidewiki)