“…An RFID reader would scan RFID tags, which consist of a computer chip coupled with an antenna. Data stored on the chip transmits wirelessly through the antenna to an RFID reader operating on the same frequency as the antenna. Many retailers use RFID tags to track products in transit; libraries use RFID tags to track books. RFID tags are also used to track humans or animals for scientific and medical purposes.”

Read more:

http://www.wired.com/gadgetlab/2009/11/iphone-rfid/