A device ID (device identification) is a distinctive number associated with a smartphone or similar handheld device. Device IDs are separate from hardware serial numbers.
Every Apple iPhone, iPod touch and iPad has a unique device ID number associated with it, known as a Unique Device ID (UDID). Apple's device ID is a 40-digit sequence of letters and numbers. Customers can access their device ID numbers in iTunes or by downloading a free app from the Apple App Store.
Android device IDs are determined during the device's first boot. Users can access their devices' randomly generated Android IDs by downloading a free app from Google Play. These device identification numbers should remain constant for the lifetime of the device unless a factory reset is performed.