A "denial-of-service attack" or DoS attack is a cyber attack on a website. During a DoS attack, a website is flooded with phony requests. Due to the massive spike in website traffic, the website simply cannot keep up and legitimate users are unable to log in.http://www.us-cert.gov/cas/tips/ST04-015.html
The social networking website Twitter was crippled by a denial of service attack on August 6, 2009. http://news.cnet.com/8301-13577_3-10302636-36.html On the same day, Facebook was experiencing similar problems. http://voices.washingtonpost.com/fasterforward/2009/08/yes_twitters_down_and_facebook.html?hpid=news-col-blog
