Passive RFID tags have an antennae or receiving coil, which collects and stores energy transmitted by the reader. The tag then emits a short burst of energy, encoded with the critical information it is communicating. Passive tags only transmit when they have been energised by a reader.
Passive tags tend to be cheaper and smaller, and their life is only limited by the reliability of their components. They can be expected to function for decades. The range at which the tags can be read depends on their frequency of operation, and the power of the reader. Range can vary from less than a centimeter to several meters.
The ability to read a tag is affected greatly by the reader. Readers can be fixed, or integrated with a portable handheld computer. In mobile applications some readers are still relatively bulky and large, with the transmitting and receiving antennae being three or four inches high and just as wide. Others are smaller, sometimes with a reduced read range as a consequence.