A Denial of Service (DoS) attack or Distributed Denial of Service (DDoS) attack is based upon flooding key servers with useless traffic. The server has to spend so much time dealing with this traffic that users are denied normal service.
There are different types of DoS attack, eg: the Ping of Death. These attacks exploit loopholes in TCP/IP (Transmission Control Protocol/Internet Protocol), the set of rules which is the basis of the Internet.
For all current DoS attacks there are software patches and fixes that can be installed by system administrators to limit or prevent attacks on their systems. However, just like viruses, new DoS attacks are constantly being developed by hackers and other Internet criminals.
In January 2011 detectives from the Metropolitan Police Central e-Crime Unit (PCeU) arrested three teenagers for offences under the Computer Misuse Act 1990. The arrests were in relation to denial of service attacks on firms, including Amazon, PayPal and MasterCard, that withdrew support for Wikileaks after its controversial release of classified US diplomatic data.