Transmitters & Receivers are usually abbreviated TX & RX. Most work on 2.4GHz band but some more advanced systems also work on 5.8GHz, the TX is held in hand to control the model and the RX is fitted into the model. When the sticks and switches are moved on the ground TX the signal is sent to the on board RX via radio waves, the models controller or servos etc then turn this signal into movement to control the model. More and more radio systems have telemetry which means that live data can be beamed back to the handset to inform the pilot of signal and battery strength etc