The so-called "CryptoLocker virus" is an example of ransomware, a class of malware that, once it has infected a particular computer system, restricts access to that system until the user pays a ransom. CryptoLocker is a particular form of ransomware known as cryptoviral extortion, a scheme in which key files on the system's hard drive are encrypted and thus rendered inaccessible to the user unless and until that user pays a ransom to obtain a key for decrypting the files.
The CryptoLocker worm is generally spread via drive-by downloads or as an attachment to phony e-mails disguised as legitimate messages from various business, such as fake FedEx and UPS tracking notifications. When a user opens such a message, CryptoLocker installs itself on the user's system, scans the hard drive, and encrypts certain file types, such as images, documents and spreadsheets. CryptoLocker then launches a window displaying a demand for ransom (to be paid in less-traceable forms such as Bitcoins and Green Dot Moneypaks) and a countdown timer showing the date and time before which the user must submit payment in order to obtain the decryption key before it is destroyed.