Card skimmers, in some respects, are rather impressive devices. Designed to be invisible, they can be installed by criminals on ATMs, gas pumps, or essentially anything else with a card slot. The principle is easy…your card gets scanned when you insert it into the machine and generally a camera will be located nearby to record your PIN when you type it in. With this information, a criminal is able to simply reproduce your card for his or her own use.
As far as I know I’ve never actually seen one. Just be safe and be observant…if a machine doesn’t look quite right then go somewhere else.