A Distributed Denial-of-Service (DDoS) attack is one in which a multitude of compromised systems attack a single target,
thereby causing denial of service for users of the targeted system.
The flood of incoming messages to the target system essentially forces it to shut down,
thereby denying service to the system to legitimate users.

examples..

For example, sending an extraordinary amount of electronic mail to someone could fill the computer disk where mail resides. This means that people who use the computer with the full disk cannot receive any new email until the situation changes. While this is an older style of DoS attack, it is still popular today.
Examples of frequently attacked services are the
World Wide Web1,
File sharing services ,
The Domain Name Service

For example, by launching a DoS attack on a popular merchant during a high sales period, the intruder affects not only that merchant, but everyone who is then unable to buy their products.