A Distributed Denial of Service (DDOS) attack occurs when criminal gangs flood a website or server with requests so that the server is unable to respond to the volume of requests made.
In a DDOS attack, criminal gangs use software that they have installed on the computers of members of the public who have not protected their computers with firewalls and anti-virus software.
The attack is launched from many of thousands of computers, which are used at different times to send different types of requests. This makes it difficult for a server to determine which requests are genuine and which are malicious. As the attack can change over time, it is difficult to eliminate.