The best known example of the kind of malware known as ransomware. CryptoLocker encrypts files on your computer and then demands that you send the malware operator money in order to have the files decrypted.
According to FBI estimates, CryptoLocker had more than 500,000 victims between September 2013 and May 2014. Around 1.3 percent paid to free their files, earning the malware makers around $3 million. The criminal network was smashed by authorities and security researchers in May 2014 and a tool put online to decryt victim's files for free.