RFID is widely used technology probably because of its simplicity not only in usage level, but also in its technology. To make RFID work, you don’t need hi tech. Check out this project made by sketchsk3tch who managed to spoof a simple 125kHz RFID tag.
He used only few components (transistor, resistor, and capacitor) and Arduino. Antenna was made by winding wire on a toilet paper drum. He used simple RFID reader to test things out. Once tag is assembled it is controlled only with single Arduino pin which simply sends tag code by tuning antenna coil. There are few shortcomings in this prototype like tag isn’t synchronized with reader – it works on its own freq what leads to some miss readings. Anyway project is more fun than use and it gives clear idea on how RFID tags work.