CryptoLocker is a new, and particularly malicious, breed of ransomware discovered only a couple months ago. CryptoLocker is a Trojan horse malware that encrypts and restricts access to the infected computer while demanding payment for the recovery and decryption of files, typically within a 72-hour deadline. CryptoLocker primarily infects computers (running Microsoft Windows) via malicious attachments found in phishing emails, which when opened, immediately infect the host computer.
But CryptoLocker is especially fierce because these phishing emails are designed to look and feel like an email from a trusted organization. They may be disguised as a harmless email from your bank urging you to check your account information by opening the attachment. They may even ask you to input a secure password to open the attachment, perhaps seeming more legitimate to the average person, but still as destructive.