Quartz is a type of crystal used in watches, computers, and other devices to keep time. The quartz crystal vibrates or ticks an exact 60-seconds per minute when electricity is applied to it. The picture shows an example of a type of crystal you'll find in your computer. This example is of a 24.576 MHz crystal found on a computer sound card.
Depending on the quality of the quartz crystal and other factors, the quartz crystal may not properly tick exactly 60 seconds each minute, causing a computer to not keep the proper time.