This is a video of a person who discovered a "skimmer" on an ATM in Vienna. The purpose of this device is to read the information encoded on your credit/debit card and transmit it to another party. As the sophistication of criminals looking to steal your information increases, detecting these types of devices will become more difficult. Here are some hints to looks for:
The bottom line is if it looks funny to you, you might want to find another ATM.