Keeping time
We measure the passage of time by counting oscillations. Any system that oscillates regularly provides the basis of a clock. In a quartz clock the oscillator is a sliver of quartz (much thinner than the big chunks shown above), which oscillates (expands and contracts) at a frequency determined by its size. The most common quartz oscillators vibrate 32,768 times per second. Some have oscillation frequencies in excess of 107 Hz. What is special about quartz? It is piezolectric: this means that an electric field can stretch it and thus initiate (and sustain) its vibration.
If you want to find out more about the guts of a quartz clock take a look here.