Transponder… it’s such a weird sounding word, but it’s pretty simple to explain: it sends a signal when it receives a signal (http://dictionary.reference.com/browse/transponder?s=t)
So, when you roll up to your EZ-Pass or Fastrak lane, a transponder reader gives your toll tag a signal to have your tag give it a signal in return so the network can process your vehicle information. All of this signal is sent via radio waves. (More info here: http://auto.howstuffworks.com/e-zpass2.htm)
The transponder receives power only when it goes through a reader, which makes it a passive RFID chip (radio frequency identification) (http://electronics.howstuffworks.com/gadgets/high-tech-gadgets/rfid3.htm).