What is a DDoS attack?

A DDoS or Distributed Denial of Service attack uses hundreds of infected systems, focussing their requests onto a service in order to prevent the handling of genuine requests. This is known as flooding.

Often, the same request is repeated by many machines thousands of times a second.

Notable DDoS attacks include ones carried out late last year by the hacker group Anonymous against PayPal, Mastercard and Visa as part of their 'operation payback' campaign in support of Wikileaks.