RFID tags work by absorbing the radio frequency energy from a scanner and using that energy to respond with data. The simplest passive tag consists of an antenna and chip containing encoded data, and its only power source is the electromagnetic energy it absorbs from the scanning pulse. These tags are cheap and easy to manufacture, but their effective range for scanning is extremely limited.
If a greater scan range is required, manufacturers can include a power source in the RFID tag. These active tags have a range that is only limited by the strength of their internal power source. A simple low-voltage battery can produce a pulse readable from anywhere inside a building, while a more powerful source is capable of creating RFID pulses detectable at a range of hundreds of yards or more. Active tags can also be set to send out regular pulses instead of waiting for a scanning carrier wave, and are used in tracking everything from stolen merchandise to endangered animals.
The simplest RFID tags contain only an encoded identification number unique to the tag, but more expensive and complex tags may feature rewritable memory, or a chip that can be rewritten a single time by the end user to store information.