A remarkable find was incurred by Pierre and Jacques Curie in the 1880's.  They had noticed that

1. Applying pressure or mechanical stress on certain natural nonsymmetrical crystals produced an electrical charge in direct proportion to the pressure

2. They also found that when those same crystals where subjected to an electric field, they expanded or contracted in direct proportion to that electrical field.

These collective properties are today go by the symantec "Piezoelectric effect".  As one may imagine, components exhibiting these controlled mechanical and/or electrical properties could be very useful in a wide variety of goods and/or devices.  They (Piezos) can serve as durable electromechanical transducers converting mechanical energy into electrical and similarly converting electrical energy into mechanical.

Diagrams/Overview of the Piezoelectric Phenomenon

Above the Curie Temperature, the crystal structure is cubic and has no electric dipole movement.
However, below this temperature the positively charged Ti/Zr ion shifts from its central location along one of several allowed directions.  This slightly distorts the crystal lattice into a perovskite structure (a tetragonal/rhombohedra shape), and produces an electric dipole with a single axis of symmetry.
Immediately after sintering, groups of molecular diploes align within small areas, or domains, to form large dipole moments.  PZT is made up of many such domains; however, as they are randomly oriented, their net electric dipole is zero.
If PZT is subjected to a large electric field at elevated temperatures, the domain diploes align in the allowed direction most closely in line with the field.  This process is called Polarization and causes the PZT to exhibit the Piezoelectric Phenomenon.  The diploes will maintain this orientation even after the dc field is removed (remnant polarization), a necessary condition for the Piezoelectric behavior of ferroelectric ceramics.