Skimmers are devices that are attached to bank machines and when an unsuspecting victim puts their card in it steals the cards details. They use to be easily noticeable if you knew what to look for, however these days they are becoming virtually impossible to spot due to becoming smaller and more advanced. Researchers at the European ATM Security Team have found skimmers that fit neatly into card slots and do a good job of hiding the rest of the equipment needed to steal your info. Check out the example below which features a skimmer that's virtually invisible and features a hidden camera to record PIN code entries.
Others include cards that captures card information through audio, scanners that are transparent to be even more invisible and high quality replacement card dashboards that look just like the real thing.
While you can't really be 100% using any cash machine, it's always best to cover your PIN, even if nobody is around, just in case a hidden camera is watching. Check out more information at Krebs on Security.