"Denial of Service." This is a term we should all become familiar with. It sounds like a pretty harmless thing, but it's the term technologists use to describe very serious attacks on internet infrastructure that have potential to seriously disrupt commerce, government, and crisis communication infrastructure.
A denial of service attack is what silenced twitter on Thursday, August 6. The attack on Estonia in April of 2007 was also a denial of service attack.
The WSJ had a good opinion piece about the state of these online attacks. Quoting: "As the attacks against Estonia show, the task cannot be delayed -- the increasing sophistication and accessibility of malware means these problems will only become worse. The future of the Web is at stake."