The Apple Unique Device Identifier, commonly referred to as UDID, is a code of 40 characters having numerals and alphabets, unique to a particular Apple device. It is like the social security number of your apple product. Using this code you can install beta versions of your app, without having to go through the Apple App Store.
Here is how you can find the UDID of your Apple device:
- Launch iTunes
- Plug in your Apple Device
- Click on device name on the iTunes menu
- Click on Serial number and Viola! Your UDID appears
Until quite recently apps used to monitor the UDID of users for marketing. App Developers used to sell UDIDs to ad agencies, which used them to analyze what kind of ads appeal to each user, thus making their marketing process a lot more streamlined. But, your iPhone probably contains a lot of personal information about you. Third party applications used to have a large database of the UDIDs of their users, giving them access to a lot more information than they should be privy to. Due to the heavy protests against this, Apple has taken a strong stand against such tracking and has threatened to ban any app which fails to adhere their privacy norms.
About a year ago, Apple told its developers to move to “Core Foundation Universally Unique Identifier”, supposedly a safer alternative, as it was planning to scrap the UDID system. As Apple has failed to implement all the functionality of UDID in it, developers are still dragging their feet over it. UDID remains the only way to download apps onto your Apple device for testing without having to put it up on the Store.