A DoS attack is designed to hinder or stop the normal functioning of a web site, server or other network resource. There are various ways for hackers or virus writers to achieve this. One common method is simply to flood a server with more network traffic than it is able to handle. This prevents it from carrying out its normal functions and in some circumstances crashes the server completely.

A DDoS attack differs only in the fact that the attack is conducted using multiple machines. The hacker or virus writer typically use one compromised machine as the ‘master’ and co-ordinates the attack across other, so-called ‘zombie’, machines. Both master and zombie machines are typically compromised by exploiting a vulnerability in an application on the machine, to install a Trojan or other piece of malicious code.