Transducers are electro-mechanical devices that convert electrical energy to mechanical energy or vice versa. A well known and widely used transducer is the so-called "voice coil" transducer that is based on electromagnetic principles and finds applications in speakers and sub-woofers.
Electrical energy is converted into motion pushing a membrane and thus generating sound waves. Piezoelectric transducers can also be used in speakers, mostly for the high-frequency range.
More sophisticated applications are found in medical devices as ventilators and nebulizers and in Intensive Care devices for ultrasonic metering. Piezo transducers were even used on the Mars Rover to sort particles!
More down-to-earth applications are found in electric guitar pickups, to convert mechanical energy into electrical energy, then amplified by an electric circuit and reconverted to mechanical energy by a loudspeaker.
Modern automobiles have hundreds of transducers, both sensors and actuators. Every park sensor uses a piezoelectric ultrasound transducer to measure the distance, and in turn, piezoelectric transducers are also often used to alert the operators of an obstacle with a buzzing sound.