"The FBI has not requested this information from Apple, nor have we provided it to the FBI or any organization. Additionally, with iOS 6 we introduced a new set of APIs meant to replace the use of the UDID and will soon be banning the use of UDID" - Natalie Keriss Apple Spokeswoman
Unique Device Identifiers, or UDIDs, are somewhat similar to a serial number or a social security number for devices. It is comprised of about 40 alphanumeric characters that basically identifies you as the device owner. It could be used for targeted ads and marketing analytics, or it can be utilized for development purposes like app and software distribution.