UDID stands for Unique Device Identifier. It is a 40-character long series of numbers and letters used by Apple and developers to uniquely identify your iPhone, iPod touch, and iPad. The UDID is being used by developers to register devices for testing their apps. Only on registered devices developers can run apps that are not in Apple's AppStore yet.
To find the UDID for your iPhone, iPod touch, or iPad:
- Launch iTunes and connect your device.
- Choose your device in the left pane. The right pane will display information about your device (ET iPhone in this example).
- Click on Serial Number to reveal the UDID.
- Copy the UDID to your clipboard by choosing Edit → Copy Identifier (UDID).
- Paste the UDID into a document or an email (You should be able to paste it by selecting Edit → Paste).