What is a UDID?
A UDID, or “Unique Device Identifier,” is simply a specific serial number for your iDevice. There are multiple advantages to having this device-specific serial, but it mainly serves as a way to document the existence of your device and to track it.
Each iPhone or iPod Touch has a Unique Device Identifier (UDID), which is a sequence of 40 letters and numbers that is specific to your device. Your UDID will look something like this: 2b6f0cc904d137be2e1730235f5664094b831186.
How do I locate the UDID?
Here are the steps to locate your UDID using iTunes.
In the left panel, click on your Apple device to view more information on the device, including its name, capacity, software version and serial number.
Click on the Serial Number, and the Identifier (UDID) will appear.
Select Edit > Copy in the iTunes menu to put the serial number on the Clipboard. You can then paste the serial number into an email or webpage (for example, for service or registration).