Distributed Denial of Service
Distributed Denial of Service, or DDoS, refers to an Internet-based attack that intends to block a service from its intended user for any variety of reasons. For example, to block access to a website for all users wishing to visit it, a DDoS attack could involve bombarding the server with communications requests, which if frequent enough can bring a server to a halt, or at least make it operate extremely slowly.
Often, DDoS attacks are carried out with botnets, which are a wide collection of hacked computers regularly connected to the Internet which can be controlled by an attacker. These machines often carry exploitable Trojan horse infections and can be given any number of commands from malicious individuals. Needless to say, DDoS attacks are against the Acceptable Usage Policies of virtually every Internet Service Provider in the world and are a violation of the Internet Architecture Board's proper use policy.
Most often DDoS attacks are carried out against particular targets that would be in constant usage, such as banks or credit card payment gateways, but sometimes attacks have been focused on root nameservers, which could have terrible consequences for how the Internet performs. Some DDoS attacks are politically motivated, such as a bunch of attacks against U.S. government websites by Chinese hackers several years ago when tensions between the countries grew.
Others are philosophically motivated, such as attacks on the Church of Scientology websites in 2008 following the institutions targeting on YouTube videos in a DMCA request campaign to remove secret videos that had leaked.