These systems have a mainly industrial usage. They are being used to control pipelines’ pressure or motor work rates on industrial factory floors. Typical environments can be oil pipelines and power-plants, factories etc.
In order to gain access to the network, Stuxnet exploits a first zero-day vulnerability in the the Windows OS. Apparently there are 4 such vulnerabilities in Windows, the second one of which allows the worm to spread across the network. After locating a machine with Scada, the worm reports back to the attacker via a command-and-control system. The last two vulnerabilities are the ones used by the worm to acquire system level privileges.
It is rumored that the complexity of the worm may prove that state-sponsored hackers are behind this mall-ware, although no firm evidence of this can be found. Iran has been the main area of Stuxnet infestations so far.