You’ve probably never seen an RFID tag, but chances are there’s one within 10 ft. of you. An RFID tag–it stands for radio-frequency identification–is a tiny device consisting of a very small chip attached to a very small antenna. The chip is no great shakes, but it doesn’t have to be; it just stores one very large number. When the RFID tag hears a particular radio signal, it responds by broadcasting back the number in its chip. That’s its entire purpose in life. Some prototype RFID tags are about the size of a grain of ground pepper.
http://www.time.com/time/magazine/article/0,9171,1673283,00.html