Passive RFID Tags 
Passive RFID tags have no internal power supply. The minute electrical current induced in the
antenna by the incoming radio frequency signal provides just enough power for the CMOS
integrated circuit in the tag to power up and transmit a response. The lack of an onboard
power supply means that the device can be quite small: commercially available products exist that can be embedded in a sticker, or under the skin.
As of 2006, the smallest such devices measured 0.15 mm × 0.15 mm, and are thinner than a sheet of paper (7.5 micrometers). [2] The lowest cost EPC RFID tags, which are the standard chosen by Wal-Mart, DOD, Target, Tesco in the UK and Metro AG in Germany, are available today at a price of 5 cents each. The addition of the antenna creates a tag that varies from the size of a postage stamp to the size of a post card. Passive tags have practical read distances ranging from about 10 cm (4 in.) (ISO 14443) up to a few meters (EPC and ISO 18000-6) depending on the chosen radio frequency and antenna design/size. Due to their simplicity in design they are also suitable for manufacture with a printing process for the antennas.
Non-silicon tags made from polymer semiconductors are currently being developed by several companies globally. Simple laboratory printed polymer tags operating at 13.56 MHz were demonstrated in 2005 by both PolyIC (Germany) and Philips (The Netherlands). If successfully commercialized, polymer tags will be roll printable, like a magazine, and much less expensive than silicon-based tags. The end game for most item level tagging over the next few decades may be that RFID tags will be wholly printed – the same way a barcode is today – and be virtually free, like a barcode. However, substantial technical and economic hurdles must be surmounted to accomplish such an end: hundreds of billions of dollars have been invested over the last three decades in silicon processing, resulting in a per-feature cost which is actually less than that of conventional printing.
Active RFID Tags
Unlike passive RFID tags, active RFID tags have their own internal power source which is 
used to power any ICs that generate the outgoing signal. Active tags are typically much more
reliable (e.g. fewer errors) than passive tags due to the ability for active tags to conduct a “session” with a reader.
Active tags, due to their onboard power supply, also transmit at higher power levels than
passive tags, allowing them to be more effective in “RF challenged” environments like water (including humans/cattle, which are mostly water), metal (shipping containers, vehicles), or at longer distances.
Many active tags have practical ranges of hundreds of meters, and a battery life of up to 10 years. Some active RFID tags include sensors such as temperature logging which have been used in concrete maturity monitoring or to monitor the temperature of perishable goods.
Other sensors that have been married with active RFID include humidity, shock/vibration, light, radiation, temperature and atmospherics like ethylene.
Active tags typically have much longer range (approximately 300 feet) and larger memories than passive tags, as well as the ability to store additional information sent by the transceiver. The United States Department of Defense has successfully used active tags to reduce logistics costs and improve supply chain visibility for more than 15 years. At present, the smallest active tags are about the size of a coin and sell for a few dollars.
