Ad Hoc distribution allows you to share an application with up to 100 iOS devices that must be registered by developers in their iOS Provisioning Portal (with a limit of 100 devices per year!). If you want to distribute your Apps to thousands of users you must submit them to the App Store, even if they are free, but Ad Hoc distribution is still pretty useful for testing purposes. You may want to share your App with betatesters before setting it free in the Wild!
In order to register a device for Ad Hoc applications developers need the unique identifier (UUID or UDID) of the iOS device where the App will be installed. How to find it?
1- Start iTunes
2- Connect your iOS device to your computer and wait for it to be listed under ‘Devices’
3- Click on your device. On the right side of the screen you’ll see its picture and, among other things, its serial number. That’s NOT the UUID. But click on it!
4- When clicked, the serial number turns into a huge alphanumeric string (40 characters). That’s the UUID/UDID.
5- Don’t try to select it with your mouse, you can’t! But you may still copy-paste it by pressing CMD-C, CMD-V (or Control if you are in Windows). Do yourself a favor and don’t do it by hand… although I guess we all did the first time.