A botnet is a bunch of computers networked together and run a program that the computer users don’t know is running. These nets are controlled remotely and usually used to do bad things, like spam the crap out of the Internet about fake insider stock market information, advertise nearly-free medication or just outright steal banking and identity information.
They’re difficult to sniff out and even more difficult to shut down. Explains Ars Technica:
The result is that while efforts such as Microsoft’s disruption of the Waledac and Rustock botnets weresuccessful, they were far from perfect. These efforts left the malicious software running on the infected PCs—they just removed the command and control servers, the centralized machines that tell the botnet what to do. Should the bot herders regain control of the domain names or IP addresses used by the command-and-control servers, the infected machines will be able to successfully connect to them, and the networks will once again spring into life.
But! There’s hope. Hit the article for the whole story.