Bluetooth is a short-range wireless standard for connecting devices within a few to tens of meters. Developed by Ericsson in 1994, it's named after 10th-century Danish King Harald Bluetooth, who unified different tribes - symbolizing the technology's goal of connecting different manufacturers' devices.
The most common use is connecting wireless earphones and speakers. After initial "pairing" (authentication), devices reconnect automatically. Keyboards, mice, game controllers, and smartwatches also use Bluetooth.
Newer Bluetooth versions offer better power efficiency, speed, and range. Bluetooth 5.0+ supports up to 200m range (theoretical) and double the transfer speed. However, walls and bodies significantly reduce range, and interference can occur in crowded areas since Bluetooth shares the 2.4GHz band with Wi-Fi.