RFID tags are generally small devices that respond to an RFID reader’s interrogation via radio frequency. Tags vary in terms of memory, the range over which they can be read, the level of read and write capabilities, and the availability of other computational functions. The tag can hold just a product’s serial number all the way up to a mass of information about the product and its history.
Passive RFID: A passive RFID tag has no battery. It harvests all its power from the radio interrogation of the RFID reader, including enough power to respond. This yields fairly low-cost devices, but they can be read only at a fairly short range (about 20 feet in the best operating conditions).
Active RFID: Active tags have batteries attached to them so that they can respond to a reader with more power. They are much more expensive than passive tags but have a much-greater range (up to 300 feet).
Smart Cards: Contactless smart cards are often referred to as RFID tags. From a technical perspective, they are just a specially packaged form of RFID tags.
Sensors: RFID tags don’t tell you anything about their environment. They can only tell you what is in the memory of the tag. However, a common application of RFID is to attach the tag to a sensor that can fill the tag’s memory with data. This is then communicated to other systems through RFID protocols. The sensory technology, while intertwined with RFID, is independent.