RFID Tags:
RFID tags are essentially microchips, the tinier the better. These chips act as transponders (transmitters/responders), always listening for a radio signal sent by transceivers, or RFID readers. When a transponder receives a certain radio query, it responds by transmitting its unique ID code back to the transceiver. Most RFID tags don't have batteries. Instead, they are powered by the radio signal that wakes them up and requests an answer.