The function of the FM transmitter is to send an incoming wireless sound signal through the FM net. The signal can be received by most radio receivers, new as well as old, for exampel a car stereo.
The sound signal is foremost designed to be sent from portable sound systems like MP3, CD and MD players, but is compatible with most sound units with a suitable audio output. Some FM transmitters connects through a cable via the headset input, while some models are connected via a special link suitable especially for Ipod.
FM transmitters are usually battery operated, but there are models available that can be operated by the 12V connection (cigarett lighter outlet) in cars.