RFID Tags come in three basic varieties: passive, active, or semi-passive or battery-assisted. RFID Tags and Readers are available in a range of different frequencies with 125kHz, 13.56MHz, 868MHz and 915MHz being the most common frequencies.
Passive RFID Tags: A Passive RF Tag requires no internal power source and generates no RF signals itself. Passive RF Tags are powered by readers when they are in range. They inherit their frequency accuracy and stability from the RFID reader to which they are responding. Passive RFID tags have practical read distances ranging from about 10 cm to several meters.
Active RFID Tags: An Active RF Tag has its own internal power source, which is used to power the circuits and to generate RF. Active RFID Tags have practical read ranges of tens to hundreds of meters, and battery lives of up to 10 years. In general, Active RFID Tags are considerably more expensive than Passive RFID tags. Active RFID Tag readers, however are relatively low cost.
Semi-passive (Battery Assist) RFID Tags: A Semi-passive RF Tag is similar to an Active RF Tag in that it has its power source. Semi-passive RFID Tags, however, do not generate RF as is the case with Active RFID Tags, but merely reflect the RF received for the RFID reader back to the reader as with a Passive RFID Tag.