CryptoLocker is a malware variant known as ransomware that targets Windows Operating Systems. It will sniff out specific file types on your system and encrypt them. It will then display a CryptoLocker payment program that prompts you to send a payment to get the decryption key in order to recover the files.
How is CryptoLocker installed?
CryptoLocker is obtained via a phishing emails with malicious attachments. The user must open the attachments on the email for the ransomware to be installed.
CryptoLocker can find and encrypt files located on local drive and attached drives, such as USB drives, shared network drives, external hard drives, network file shares, and even some cloud storage drives. Not only can this affect the files of the user, but it could affect the files of an entire department if the files on a shared network resource are encrypted.
From the US-CERT website: