RFID uses the same principles as a grid dip oscillator. In the RFID Tag, there is a tuned circuit, and a small low power chip that is powered by the RF the tag receives. The RFID reader puts out RF at the frequency the tag is tuned to. When the RFID tag is in close enough range, the tuned circuit absorbs some of this rf, which the reader can detect. Then, the RFID tag uses a transistor to short the tuned circuit. This is how it sends it's data back to the reader. The exact frequencies and methods vary from manufacturer to manufaturer.