The first type of RFID available is passive RFID. Passive RFID uses high-power readers that send out a low-frequency, high-power RF signal to battery-free tags. The antenna in the tag is woken up by the amount of energy flowing to it, which wakes up its circuit. 

The tag then transmits a coded message back to the reader at a different frequency. Passive RFID technology is often used for inventory tracking and to deter theft.