Answer of What is an RFID tag :
In general, an RFID tag consists of an application-specific integrated circuit (IC) with memory chips used for data storage and an antenna that can be mounted on various substrates. Each element of an RFID tag is selected for optimum efficiency for the application.
Physical sizes can range from as small as a thumbnail to as large as a brick. Most of today¡¯s tags offer up to 128 bits of memory and read ranges up to 20 feet, dependent upon the application. The term ¡°tag¡± and ¡°transponder¡± are synonymous.