Sony Pictures
Here's how it works.
It sends out emails designed to look as though they were sent legitimately from news sources like The Guardian and others. A population of people end up clicking on a link that takes them to a shady site that scans their computer for vulnerabilities. It works against Windows, OS X and Linux systems, and there may be iOS and Android versions on the way.
Once infected, a computer surrenders pretty much any info the malware wants. It can collect "network traffic, keystrokes, Skype conversations, analyze Wi-Fi traffic, PGP keys, fetch all information from Nokia devices, screen captures and monitor all file operations."
And lest you need a reminder, no one knows where it came from.
If you want to dig into the nitty-gritty of it all, Kaspersky Labs released this extensive report on Careto that gets into a lot of the scarier technical details.