A transducer converts one kind of energy to another. For example, a microphone receives acoustic energy (sound pressure waves) and converts them to electrical energy (electrical signal). One more example of a transducer: a speaker converts an electrical signal into acoustic pressure waves.
A transponder recieves an "interrogation signal" and responds with a preset code or signal. Airliners all have transponders; when an air controller sends a "who are you" signal, the transponder automatically resonds "I am XYZ flignt numer 123, at 25,000 feet altitude, and heading 270 degrees". The pilot is not distracted by having to transmit all this data and can concentrate of flying the aircraft.
Transponders can also be found in harbors mounted on bouys. Ships will ping on them, and they respond. The ship calculates the distance to the bouy by measuring the time it takes for the round-trip of the signal.
