What is the UDID?
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. It’s like a serial number but much harder to guess. It will look something like this:
Why do we need the UDID?
Your iPhone can only install programs that are approved by Apple. Applications in the App Store have been approved by Apple for general distribution, but beta customers get to try the app before it’s in the store. We register your UDID with Apple so they can approve our application especially for your iPhone.
How do I get my UDID?
You can copy/paste your UDID from iTunes or email it directly from your device by using a free app from the App Store.
Email Using the Free App
Install and run Ad Hoc Helper.
It will create an email with your UDID. Send it to yourself.