A keylogger is a type of surveillance software or hardware that secretly records every keystroke made on a computer or mobile device. Cybercriminals often use keyloggers to capture sensitive information such as passwords, credit card numbers, and personal messages without the user's knowledge. While keyloggers can have legitimate uses, such as monitoring employee activity or parental control, they are most commonly associated with malicious intent and data theft. Protecting against keyloggers involves using updated security software, being cautious with downloads, and regularly monitoring device activity.