Radio Frequency Identification (RFID) is a technology that uses radio waves to passively identify a tagged object. It is used in several commercial and industrial applications, from tracking items along a supply chain to keeping track of items checked out of a library.
There are two main kinds of RFID. One is low frequency (below 1 GHz), and uses weak transformer coupling in the very near field to get power into the chip and to detect changes in its impedance that it is programmed to make
when it receives power. The other is far-field and higher frequency and uses transmitting and receiving antennas, not a transformer. Tags can be active, which increases their range, but most are passive, with no power on-board.