Skimming is the theft of credit card information used in a legitimate transaction.
The fraudster uses a small electronic device or skimmer to swipe and store credit card numbers. Instances of skimming have been reported where the fraudster has illicitly installed a device or group of devices on an ATM.
Recently-made ATMs often run a picture of what the slot and keypad are supposed to look like as a background so you can identify if any foreign devices are attached.