The name of this attack, “denial of service” (DoS), refers to the intended result. Also known as a “distributed denial-of-service attack (DDoS attack), these types of attacks attempt to make a Web site unresponsive, in other words to crash it. Of course the target can be any Internet site or service but the most likely targets are high-profile servers.
The perpetrators of these attacks usually flood the target with massive requests. The servers get overwhelmed and stop responding to legitimate requests. A “successful” denial of service attack forces the target computer to either reset or it consumes so much of the server’s resources that it can no longer provide service to its customers. These attacks are malicious, malevolent, and unethical. Perpetrators may be prosecuted under the laws of some countries.
For more technical information go to Wikipedia page Denial Of Service Attack.