http://www.rfidjournal.com/article/articleview/208 has a glossary of terms commonly used when discussing RFID.

"RFID tag: A microchip attached to an antenna that is packaged in a way that it can be applied to an object. The tag picks up signals from and sends signals to a reader. The tag contains a unique serial number, but may have other information, such as a customers' account number. Tags come in many forms, such smart labels that can have a barcode printed on it, or the tag can simply be mounted inside a carton or embedded in plastic. RFID tags can be active, passive or semi-passive."

There are concerns about how this technology can be used to track people's movements and otherwise invade privacy.