CryptoLocker was a ransomware trojan which targeted computers running Microsoft Windows. CryptoLocker propagated via infected email attachments, and via an existing botnet; when activated, the malware encrypts certain types of files stored on local and mounted network drives using public-key cryptography, with the private key stored only on the malware’s control servers. The malware then displays a message which offers to decrypt the data if a payment is made by a stated deadline, and threatened to delete the private key if the deadline passes.

Source: https://en.wikipedia.org/wiki/CryptoLocker