Recent DDoS attacks on the online poker industry
For those unfamiliar with computer hacking, a denial of service attack is "a type of cyber attack in which a perpetrator attempts to temporarily or indefinitely destroy the services of a host connected to the Internet to make it impossible for the target user to use the machine or network." Resources", to Wikipedia. This is done by flooding the target network with "redundant requests" to overload the system and prevent legitimate requests from being satisfied.
According to Wikipedia, the "distributed" part of DDoS means that a large number of requests on the target come from multiple sources. "This actually makes it impossible to stop an attack by blocking a single source."
Imagine how difficult it is to fight this kind of attack, although online poker sites will definitely try to improve security to prevent it.