iBeacons are Bluetooth beacons that send out an Apple-defined Bluetooth LE advertising packet. The packet contains a unique id consisting of three parts; a UUID, major and minor.
Getting Started with iBeacon - Apple Developer (pdf)
iBeacon advertising packet structure
What is iBeacon Technology?
Choosing UUID, Major, Minor and Eddystone-UID For Beacons
iBeacon Parameters: UUID, Major and Minor
What is iBeacon?
iBeacons For Apple Passkit
Make Your Mac Advertise iBeacon
New Raspberry Pi W iBeacon Software
Beacon Compatibility