Beacons are devices that support Bluetooth low energy and allow bi directional communications with them. So, with Beacons you can take advantange of this new way of communication and solve new scenarios of Geofencing with accurracy and enable to solve several in-door localization problems.
See Beacons Scenarios to see why Beacons become important in many localization scenarios.
Technically speaking, a Beacon is in general device with at least 2.4 GHz Bluetooth 4.0 Smart (also known as BLE or Bluetooth low energy).
But it's not the same old Bluetooth (the one that required pairing and never actually worked).
It's a completely new, redesigned standard developed by Nokia™ that is now implemented in all modern smartphones like Apple iPhone™ 4S or Samsung™ Galaxy S III, and other smart devices like Google Glass™, Fitbit trackers or smartwatches.
It's called low-energy because it really is. BLE devices can run up to 2+ years with a single coin battery depending on the signal strength and how frequent they broadcast information around.
You can think about the beacon as a small lighthouse tower that's installed in a fixed location and broadcasts its presence to all the ships (smartphones) around. They could be as little as 2 inches and as far as 230 feets (approx. 70 metres) away.
Beacons external object