Cryptolocker is a trojan virus, first discovered in September 2013, that encrypts files on a local computer hard drive or mounted network drive. Once it is loaded, message is displayed to the user, telling them they must pay in Bitcoin or with a prepaid voucher by a specific deadline date to get access to the locked files. This type of ransom action places Cryptolocker in a class of viruses called ransomware.
The Cryptolocker virus is transmitted most often through e-mail as an attachment. It activates when a user reads the e-mail and tries to open the attachment. Although it is easily removed from a computer using antivirus software, the files that have been encrypted will stay encrypted.