ATM card skimming is where a thief goes to an ATM and attaches a card reader on top of the real reader. This fake reader, or skimmer, passes cards on to the ATM, but in the process it gets the card information and stores it for later retrieval. In addition, the thief installs some type of hidden camera to record videos of the cardholders typing their PINs. These two items (the card info and the PIN) are all a criminal needs—he creates a duplicate card and has the PIN to go with it. The bank thinks it’s you. What I find most interesting are the sophistication of the skimmers and how realistic they look. It can be very difficult to spot a fake. The linked article discusses card skimming in detail, and I recommend reading it.