RFID tags consist of an integrated circuit attached to an antenna-typically a small coil of wires-plus some protective packaging as determined by the application requirements. RFID tags can come in many forms and sizes. Some can be as small as a grain of rice.
There are basically two types of RFID tags, active and passive. Active tags tags have a battery, which is used to run the microchip's circuitry and to broadcast a signal to a reader have power on the chip that are generally used for tracking high-value goods that need to be scanned over long ranges, such as railway cars, but they cost a dollar or more making them too expensive for lower cost items in the supply chain. Passive tags have no battery, they draw power from the reader, which sends out electromagnetic waves that induce a current to the tag's antennas.
Tags can come be read only, write once, or fully writable. The writeable memory can be as little as 64 bits for passive tags and up to as much as 32 kB for active tags. This memory can be read or written very quickly, in some cases as fast as 200 tags/sec! Having some memory gives your process the ability to read or write any data to it that you wish. This includes test data, location data, or even the new EPC codes.