A DDoS attack occurs when multiple systems coordinate a synchronized DoS attack to a single target. The essential difference is that instead of being attacked from one location, the target is attacked from many locations at once. The distribution of hosts that defines a DDoS provide the attacker multiple advantages:
Neat reach
Attacker can force the greater volume of machine to execute a seriously disruptive attack.
The location of the attack is difficult to detect due to the random distribution of attacking systems.
It is more difficult to shut down multiple machines than one.
The true attacking party is very difficult to identify, as they are disguised behind many (mostly compromised) systems