RFID tag is a microchip combined with an antenna in a compact package, the packaging is structured to allow the RFID tag to be attached to an object to be tracked.
The tag's antenna picks up signals from an RFID reader or scanner and then returns the signal, usually with some additional data (like a unique serial number or other customized information).
RFID tags can be very small - the size of a large rice grain. Others may be the size of a small paperback book.
There are two types of tags, Active RFID tag and Passive RFID tag (Some time you could find a 3rd type called Semi Passive RFID tag)