What is Piezo sensors and Transducer? 

 Sensor is a device which detects one form of energy and converts the data to electrical energy.

Transducer is a device which converts one form of energy into another one (anyone). In fact, sensor is  a type of transducer. However, transducers also consist of devices that convert energy into other forms, such as actuators.

Piezoelectric transducers typically transmit and receive waves used for sensing. Piezo transducers convert electric charges into energy, for instance ultrasonic waves. Piezo transducer typically operates at resonant frequency with various construction options and power levels.

Piezoelectric sensors convert mechanical movements (acceleration pressure, temperature or strain) to electric signals.