Bluetooth Classic (also known as Bluetooth v2.1 + EDR) and Bluetooth Low Energy (BLE or Bluetooth v4.0) are two different versions of the Bluetooth wireless communication protocol.
Bluetooth Classic is designed for streaming audio and transferring files between two devices. It is typically used for connecting devices such as headphones, speakers, and keyboards to a computer or smartphone.
BLE is designed for low-power, low-bandwidth applications such as sensors and wearables. It is typically used for connecting devices such as fitness trackers, heart rate monitors, and smart watches to a smartphone.
For example, if you were connecting a pair of wireless headphones to a smartphone, you would use Bluetooth Classic. If you were connecting a fitness tracker to a smartphone, you would use BLE.