Cryptolocker is a type of malicious software known as ransomware. It encrypts files on a victim's computer, making them inaccessible until a ransom is paid. Typically, the ransom is demanded in cryptocurrency, making it difficult to trace. Once Cryptolocker infects a system, it displays a message informing the user of the encryption and the payment required to unlock their files. This malware often spreads through phishing emails or malicious downloads, targeting both individuals and organizations. Regular backups and security software can help protect against such threats.