A program or hardware device that captures every key depression on the computer. Also known as "Keystroke Cops," they are used to monitor an employee's activities by recording every keystroke the user makes, including typos, backspacing and retyping.
A software-based keystroke logger can also be a Trojan that was installed clandestinely with the intent to steal passwords and confidential information. See snoopware
The KeyGhost Keystroke Logger
This unassuming device is attached between the keyboard cable and the socket on the back of the computer. It captures 500,000 bytes of whatever comes across the wire and requires no software on the computer it is monitoring. The contents are retrieved when connected to another computer. (Image courtesy of KeyGhost Ltd., www.keyghost.com)