Stuxnet is one of the most famous computer worms that was discovered several years ago, in the middle of 2010. The first time when Stuxnet was noticed, it was targeting the Iranian nuclear facility with an attempt to injure the country’s uranium enrichment program and prevent President Mahmoud Ahmadinejad from building a nuclear weapon.
This is not a typical worm - instead of trying to steal credit card details, passwords or other sensitive information, Stuxnet is launched against industrial systems. It causes the centrifuges to self-destruct, creating lots of damage. According to security experts, this threat had a great potential to be used for physical destruction.
It seems that attackers have designed this digital worm with great care so that it wouldn't hit those computers and networks that do not meet specific configurations. According to some security experts, it is also set to apply some self protection tips, like easing itself on June 24 2012 and similar. The way targets get infected with Stuxnet is really unprecedented - this virus has already used four zero-day vulnerabilities. In addition, this half a megabyte in size virus can also infect systems through removable drives, such as USB drives or similar.
Iranian officials have found that an antivirus, or at least a fake copy of the antivirus that was precieved to be the real antivirus, Siemens SCADA was supplying Stuxnet with updates and code instead of detecting it and erasing it from the user's computer.
In the end of 2011, security experts noticed a new threat (duqu) that seems to be created from the same code base as Stuxnet. However, having almost identical code base, this virus seems to be released for completely different reason than its predecessor – it seems to be designed for information theft.
Other Viruses in the Stuxnet family
Video on how stuxnet infects PLCs 
Article about Stuxnet on PC MAg