A device ID (device identification) is a distinctive number associated with a smartphone or similar handheld device. Device IDs are among one of the easiest ways to identify mobile users.
Device IDs can be used to identify mobile users for applications, a well as for advertisers, marketers and services that can use the number to identify and track particular types of devices. As an example, device IDs can be utilized to show user behavior by identifiers like by region or by device. Advertisers can use this to see if users have clicked on their adds. Device IDs can be restricted in not being connected to potentially identifying information in order to follow consumer privacy laws.
Device IDs are stored on the mobile device and are separate from hardware serial numbers.
Apple device IDs
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
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 the user performs a factory reset. Google Play Developer Content Policy requires device IDs must not be connected to personally identifiable information.