Radio Frequncy Indentification (RFID) are intelligent bar codes. They came out as a step up to UPC bardcodes, which have been used since the early 1970's in order to help manufacterers and retailers keep track of their inventory through the fingerprint technology that stores readable paralell bars of binary code. The main disadvantage to this system is that the UPC barcodes are read only technology, therefore cannot send out information. RFID are able to read and write as well as modify, update, or lock the data stored within. They were originally used to track cattle, railroad cars and airline luggage, going by the name inductively coupled RFID tags which were powered by electromagnetism. Now there are active, semi-passive, and passive RFID tags. They make it possible to track merchandise for stocking and market purposes in order to see how quickly products are purchased and by whom. They have also been added to transportation devices such as highway toll passes and subway passes, making life significantly more convenient with the inventions of E-Z pass and Metrocards.
Each tag has the same basic function. The stored data within an RFID tag's microchip waits to be read when the tag's antenna recieves electromagnetic energy from an RFID reader's antenna. Using the power from its internal battery or from the reader's electromagnetic field, the tag sends radio waves back to the reader who then picks up the tag's radio waves and interprets the frequencies as meaningful data. Active and semi-passive RFID tags use internal batteries to power their circuits while the passive relies entirely on the reader to be their power source. They have 3 different storage types: read-write, read only, and WORM (write once read many) which allows some to be over written and others only to read.
http://electronics.howstuffworks.com/gadgets/high-tech-gadgets/rfid1.htm
http://www.uktelematicsonline.co.uk/html/rfid.html
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