In a DDoS attack, the attacker(s) will attempt to send a large number of requests to a target server. The combination of all these requests will saturate the server, which will not be able to handle them and will become unavailable.
This is not the type of computer attack that requires the most technical knowledge, although it can sometimes require a high level of preparation. When an attacker or group of attackers wants to attack a target, they need to have a sufficient number of machines ready to send the requests. In some cases, the attacker will use a network of ‘zombie’ computers that have been infected with malware. They use each infected computer as a source for sending requests on the day they decide to launch their #DDoS attack.
Other DDoS attacks, far more spontaneous and less prepared, have made history. This is particularly true of the attacks carried out by the Anonymous group in 2012 in response to the closure of MegaUpload. By massively distributing a tool that allowed them to send a large number of requests, the group was able to count on the support of thousands of followers of their ideology to attack and take down certain sites, including US government sites.