The asymmetric shape of the atoms of piezoelectric crystals creates positive and negative poles sensitive to electricity and deformation. This causes the crystal to produce a small voltage when it is vibrated or when enough pressure is applied to change the shape of the crystal. This effect is reversible; when an electric voltage is applied, the crystal changes shape by a small amount.
One of the best known uses of piezoelectric crystals is in a cigarette or a gas grill lighter, where pressing the button causes a spring-loaded hammer to strike the crystal. This pressure deforms the crystal and creates a high voltage that ignites the gas as the current jumps a small spark gap.
The auto focus in a digital camera also uses piezoelectric crystals, notes Green Energy Help Files. Vibrations applied at a certain frequency to two surfaces of the crystal create a force between them. One surface is fixed, and the other surface is forced to move.
Piezoelectric crystals are used in buzzers by oscillating a current through the crystal, causing it to vibrate, according to Wikipedia. The crystals are also useful in some types of actuators for aerospace applications. A voltage is applied to cause a linear deformation. Although the deformation is small, these actuators generate a large amount of force.