Answer
Low power usage is achieved because of the topology change. In classic BT, a slave device must have its receiver ON all the time to catch an incoming connection; this means current draw of 25 mA most of the time. In BLE, the slave device chooses when to advertise that it can connect. This allows it to manage exactly how much energy to expend and when. At higher levels, classic BT is mainly used to relay bulk data. BLE utilizes a small database that resides in the chip and the BLE protocol (called GATT) acts like SQL to read and write to those data records. Hence, BLE is well suited for sensor-type applications where small blocks of data need to be transferred. BLE enables up to 512-byte chunks of data to be manipulated over the air.
For more articles and definitions, see the Laird Knowledge Center.