Difference between active and passive tags:
Active RFID tags have a battery, which is used to run the microchip's circuitry and to broadcast a signal to a reader (the way a cell phone transmits signals to a base station).
Passive tags have no battery. Instead, they draw power from the reader, which sends out electromagnetic waves that induce a current in the tag's antenna. Semi-passive tags use a battery to run the chip's circuitry, but communicate by drawing power from the reader.
Active and semi-passive tags are useful for tracking high-value goods that need to be scanned over long ranges, such as railway cars on a track, but they can cost anywhere from a dollar to over $100 each, making them too expensive to put on low-cost items.
RFID systems use different frequencies:
RFID systems use many different frequencies, but generally the common are low- (around 125 KHz), high- (13.56 MHz) and ultra-high frequency, or UHF (850-928 MHz). Microwave (2.45 GHz) is also used in some applications.
Radio waves behave differently at different frequency, so choice of frequency should be based on the demands of the application.
More information about RFID is available from Gamma Solutions.