Storm isn't necessarilly the most ingenious or heinous malware circling around the net, but it is or will soon take the place of the most productive, with an estimated 1 million computers infected globally.
The nature of Storm is not widely decided upon and even though most call it a worm, other say it is a trojan. The name “Storm” is a reference to it's spam mail content, which mentions Storms. The ones that gave the name, F-secure from Finland, attribute Storm to the Zhelatin group, supposedly based in Russia.
Storm doesn't use such malicious tactics as keylogging, erasing files or stealing passwords. Instead, Storm is infamous for its frequent code updates and its ability to change the spam it produces, also it uses a peer-to-peer control structure, making it extremely hard to kill. These methods seem like a means of creating a huge botnet. And already F-secure says it is the largest in the world with around 1 million computers. Other researchers, however, say it is very hard to determine the actual number.
Storm hasn't been doing significant damage to systems, especially compared to such pieces of malware as Blaster or Slammer, no, Storm just sticks to mass-mailing and the occational denial-of-service attack, typicaly against malware researchers. The size of the Storm botnet, however, doesn't allow researchers to discard it as harmless, because should Storm's tactics change, critical networks could be attacked.