A New Attack takes down mastercard.com by Operation Payback organized under the concept of Anonymous using thousands of people voluntarily joining a botnet to direct Denial of Service Attacks at enemies of WikiLeaks.
Now, in English...
Anonymous is an adhoc protest pseudonym taken on by various people on the internet. It relates to the movie V for Vendetta but basically captures the idea of a assigning a generic identity to a faceless mass of people all acting for a common purpose. There is no known central organizing person for protests by Anonymous but its not surprising that the WikiLeaks conflict would draw people to using the identity since it involves technology, freedom of information and speech.
A Denial of Service attack (DoS) is probably the simplest form of attack there is on a website or computer system. It doesn't require any hacking or breaking of codes. The attack simply directs a large amount of traffic to a website or server all at the same time in order to overload the system and bring it down. In the simplest form this could be thousands of people agreeing to point their webrowser at the offending website at exactly some time of day. Usually these attacks are actually carried out in a more automatic way using a botnet.
A botnet is a term used to describe a network of computers usually taken over with viruses and other destructive software. A botnet might be used to scour credit card databases, attack company or government servers or host porn or marketing website content. The Operation Payback botnet is interesting because people have voluntary installed software on their computer to allow it to be used to orchestrate attacks on organizations seen at undermining WikiLeaks. People joining such a network should be aware that the actions being undertaken by those directing their computer could easily be construed as illegal, even terrorist attacks. It is not clear that individuals joining such a botnet would be free from prosecution and they cannot hope that their machine ip and identity will be hidden by the people centrally running it. However, for law enforcement authorities the truth is also that this kind of attack can't really be stopped. Just as sharing information is virally unstoppable these kind of attacks can always be reinitiated even if you take down the people centrally running this particular one. Just as legal attacks on music piracy have led to ever more subtle and unstoppable file sharing schemes like torrents, a concerted effort to restrict organizing and protest of this kind will likely only lead to evolution of more decentralized and unstoppable forms via natural selection.
Note: If there are other computer/technology terms being thrown around regarding the WikiLeaks story (or any story) and Wikipedia isn't doing it for you, then let me know in the comments and I'll do my best.