Non-technical people will imagine beacons as a small battery-powered device sticked onto a wall or place. Technical people will say it's a small Bluetooth Low Energy (BLE) device that broadcasts a small amount of data every second or so. At a hardware level, beacons are BLE devices broadcasting data using Apple's “iBeacon” protocol. At software level, beacons are messages sent by broadcasting devices that are detected and processed by receiver devices like a mobile app running on iOS. These data are considered beacons as well, depending on how you name things.