A keylogger is surveillance software that records keyboard input to monitor and track keystrokes on a computer or mobile device. Keyloggers work by capturing each keystroke like a person watching from behind, and they can send this logged data remotely or store it locally. There are hardware and software keyloggers, and software keyloggers can infect devices through links, scams, unauthorized sources, or supply chain attacks. Keyloggers are often used maliciously by cybercriminals to steal personal information, but can also be used for legitimate monitoring by employers, parents, or law enforcement. Symptoms of keylogger infection include suspicious hardware, unknown processes, device performance issues, and antivirus alerts.