Fake keyboard is placed over an ATM’s legitimate one and records the typed-in PIN. It records the PIN as you type it in and sends this information to criminals in real time. The keyboard is virtually undetectable by anyone who is not an expert, and looks exactly like the real thing. US ATM users are particularly susceptible to these type of theft, since many ATMs work on the same principle. The chip-and-PIN technology used in Europe is harder to crack so a number of US banks have started adopting it.
Source: https://thehackernews.com/2011/01/undetectable-fake-atm-keyboard-steals.html