How does a hardware keylogger work?

What will be an ideal response?

As a hardware device, the keylogger is inserted between the computer keyboard and USB port. Because the device resembles an ordinary keyboard plug and because the computer keyboard port is often on the back of the computer, a hardware keylogger is often undetected. The device collects each keystroke and the attacker who installed the keylogger returns at a later time and physically removes the device in order to access the information it has gathered.