It is like a keystroke recorder but much more sophisticated. It is a form of hacking that can monitor all keyboard activity:
“Computer criminals could soon be eavesdropping on what you type by analysing the electromagnetic signals produced by every key press.
By analysing the signals produced by keystrokes, Swiss researchers have reproduced what a target typed.”
Unlike former keystroke recorder methods, hardware does not need to be attached to the target. The hacker with the right equipment just has to be near.