Here's a bit more information from the Financial Times.

http://www.ft.com/cms/s/0/e9d3a662-c740-11df-aeb1-00144feab49a.html

The Stuxnet computer worm spreads through previously unknown holes in Microsoft’s Windows operating system and then looks for a type of software made by Siemens and used to control industrial components, including valves and brakes.

Stuxnet can hide itself, wait for certain conditions and give new orders to the components that reverse what they would normally do, the experts said. The commands are so specific that they appear aimed at an industrial sector, but officials do not know which one or what the affected equipment would do.