Kerry Sanders and Bob Sullivan, NBC News:
The UDID – which stands for Unique Device Identifier – is present on Apple iPads, iPods and iPhones, and is similar to a serial number. During the past year, researchers have found that many app developers have used the UDID to help keep track of their users, storing the data in various databases and often associating it with other personal information. When matched with other information, the UDID can be used to track users’ app usage, social media usage or location. It could also be used to “push” potentially dangerous applications onto users’ Apple gadgets.