A piezoelectric crystal is a material that expands and contracts when an electric current is applied. The piezoelectric effect converts this mechanical stress or vibration into electrical signals.

Piezoelectric crystals are used in a wide range of products and systems as either actuators or transducers.

Actuator - A device for controlling a mechanism or system.
E.g. A musical Greetings card
Pressure => Sound

Transducer - A device that converts a signal from one form to another
E.g. An electrical drum kit
Pressure => Sound