Denial of Service (DoS) attacks aren't actually designed to steal Data. They are, however, extremely effective in denying access to a particular website. Websites are designed to service traffic requests, an attacker simply generates thousands of web requests and fires them off at the target site. The volume of requests denies service to genuine queries. A Dos attack can be effectively dealt with by programming the sites router to drop all traffic from the originator meaning the requests never actually reach the server. In response to this defence hackers developed the Distributed Denial of Service attack. The attackers code is distributed about the internet using malware. When the attack commences all infected computers send thousands of web requests to the target server. Because the traffic is coming from many more sources then filtration and thereby defence agains a DDoS attack is much more difficult.