In the simplest of terms, when a piezoelectric material is squeezed, an electric charge collects on its surface. Conversely, when a piezoelectric material is subjected to a voltage drop, it mechanically deforms.
Many crystalline materials exhibit piezoelectric behavior.
A few materials exhibit the phenomenon strongly enough to be used in applications that take advantage of their properties. On a nanoscopic scale, piezoelectricity results from a nonuniform charge distribution within a crystal's unit cells. When such a crystal is mechanically deformed, the positive and negative charge centers displace by differing amounts. So while the overall crystal remains electrically neutral, the difference in charge center displacements results in an electric polarization within the crystal. Electric polarization due to mechanical input is perceived as piezoelectricity.