Bluetooth is a personal-area wireless technology in the 2.4 GHz ISM band, specified by the Bluetooth SIG. It comes in two incompatible flavors: Bluetooth Classic, built for continuous streams such as audio and serial links, and Bluetooth Low Energy, designed for devices that send small amounts of data and sleep most of the time.
In IoT, “Bluetooth” almost always means the low-energy form. Its strength is that every modern phone speaks it, so a sensor, lock or wearable can be set up and read without any extra hardware. The limits are range, typically a room or a building floor, and the lack of a direct path to the internet, which is why Bluetooth devices often pair with a phone or a gateway that relays data onward. See Bluetooth LE for IoT.
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