RFID tag is a small device with a memory chip and antenna. Their tiny form and low cost make them irreplaceable for many cases where bulky Bluetooth tags cannot help. GearEye uses thin stickers which are ideal for many small items.
The tag picks up signals from and sends signals to a reader (the Dongle). The tag contains a unique serial number but may have other information. Tags come in many forms: as a thin sticker, thicker on-metal tag, or the tag can simply be mounted inside a carton, or embedded in plastic.
See http://www.rfidjournal.com/site/faqs for more information