Cryptolocker is delivered via email.
Once the email is opened Cryptolocker runs around your disk and encrypts everything not needed to run the operating system. When the menace finishes you can no longer access any of your files.
The next insult is a window that pops-up running a countdown clock with the ransom demand. Purchase a key to get your files back or watch them flush away like a turd in a toilet. To get the key you are directed to an online virtual currency site where you must buy bitcoins in order to make the key purchase.
Why bitcoins? Because bitcoins themselves are an encrypted currency untraceable to anyone buying or accepting them as payment.
The cops think the scheme is the work of a large well organized and super tech saavy criminal enterprise in Asia or Africa. So far, these guys are getting away with the extortion undetected.