transmitters and receivers
The transmitter, the receiver and the antenna are the three main component of a radio
http://electronics.howstuffworks.com/oscillator3.htm
The above picture is a example a basic oscillator used in transmitters and receivers. It uses a capacitor and a inductor to create or detect current from the antenna.
A transmitter works by fluctuating a magnetic field through the inductor at a frequency that will cause resonance. This resonating through the antenna is what produces a radio wave.
A receiver works because when a radio signal crosses the antenna a current is created, but only the right frequency will cause the resonance to occur.