Apple’s UDID (Unique Device Identifier) is a unique series of numbers and letters assigned permanently to each iOS device (iPhones, iPads, etc.) and could be accessed by installed apps.
UDIDs have been used by app developers, mobile ad networks and marketers to identify and track specifically each device and to deduce user profiles and interests based on their app usages across many distinct apps.
Since iOS 6, Apple has progressively banned the use of UDIDs by new apps and encited developers to update old ones. Indeed, UDID tracking was criticized by Congress and Privacy advocates as users could not opt-out.
UDID is now replaced by Apple Advertising Identifier.