A keylogger is a technology that tracks and records consecutive key strokes on a keyboard. Because sensitive information such as usernames and passwords are often entered on a keyboard, a keylogger can be a very dangerous technology. Keyloggers are often part of malware, spyware or an external virus.
Techopedia explains Keylogger
There are many different kinds of keyloggers based on diverse keylogging methods. These include hardware and software keyloggers. Software keyloggers can be built into rootkits or other less detectable forms, and can infiltrate a computer in various ways. Hardware keyloggers can be fitted into the line from a keyboard to a device. Other more esoteric forms of keyloggers are based on electromagnetic emanations from hardware, which are addressed by emissions security protocols.
While keylogging is an ever-present danger in many parts of the tech world, some new technologies are keylogger-resistant. The touch screen interfaces of mobile devices often render conventional keyloggers useless, and visual methods like drop-down box selections can also provide a foil to keyloggers, given a secure data entry method for these selections. In many cases, however, keylogging can be very effective, and is becoming a serious issue for the average technology user.