The UDID (Unique Device IDentifier) is a unique identifier (40 characters) specific to Apple devices (iPhone, iPad, iPod). This identifier is used normally used by application developers to provide access to their apps before submitting them to the App Store.
In this FAQ you will learn how to find the UDID of your iOS device.
Method 1 - iTunes
Connect your iOS device to your PC.
Select your iOS Device.
Click on the Serial number and the UDID will be displayed:
Method 2 - easy-udid
Easy-UDID is an online service allowing you to easily identify the UDID of your iOS Device. From your iOS device: