UDID (Unique Device Identifier) as its name implies is unique identifier for your iphone or ipod touch. you need to supply a developer with your iPhone's UDID in order to install ad-hoc applications on your iPhone.So that developer can sign an application package and you can install it.If you are among those developer guys and interested in beta testing then go get UDID for your iPhone.

To know UDID of your iphone or ipod

Download and install the iTunes on Your Windows or Mac.

Connect your iPhone to your computer.

Wait till your iPhone appears in iTunes under Devices menu.Select it

Click the summary tab. The information including the serial number for your iPhone will display.

Click on the Serial Number field. It will change to the Identifier, and will show 40 characters- alphanumeric String that is your UDID.

Apps are available which lets you know your UDID in mail or lets you copy it.
one such application UDID sender is available at iTunes

http://itunes.apple.com/us/app/info-udid-sender/id366236549?mt=8