A computer's actions are coordinated by refers a crystal embedded in the motherboard.
To understand the oscillations of crystal, you've got to know a bit about piezoelectricity, the relationship between certain materials, electricity and pressure. Piez is Greek for pressure: pressure-electricity. When piezoelectric substances are pressurized, they produce electricity; when they're infused with electric current, they distort and pulsate in movement called oscillation.
Chew gum with crystals in the dark in front of a mirror and watch the sparks dance in your mouth. That's piezoelectricity.
Quartz is the most common mineral on earth. Silicon Dioxide — SiO2. Take a handful of virtually any earth and you've got quartz crystal in your hands. In large enough and clear enough pieces it's mistaken by children for a precious substance. Nearly all of us at one time had a collection of cloudy quartz, washed under the tap, lined up on shelves. Little did we know that we could've collected more of it bringing home buckets of sand. You can buy quartz at the store in large pieces: it's cheap, so people don't bother to polish it much. Anyone with a Paganistic bend knows its spiritual qualities.
Quartz is piezoelectric. Put it in a vice and it produces electricity. Connect it to a battery and it oscillates. By closing a circuit around it to create a feedback loop, you can make quartz oscillate to a very precise rhythm — resonance. But don't expect to produce lightning or vibrators in your garage — quartz's piezoelectric shifts are too small to notice with your primitive senses. You'd need a machine.
Many of us have seen a computer motherboard. Probably somewhere near the PCI slots you saw a small white hard object banded with silvery metal. That's a small quartz crystal, precisely cut, and fed with electrical current. Its tiny oscillations are received by traces which carry them to the board's other chips and expansion slots.
As you know, computer data is binary. So is computer activity. Think of the 'all-or-none' response of human muscle fibers to chemical transmitters: it's similar for binary operations. Work or stasis, nothing in between. Consequently, the processing power going for your music, your gaming, your Everything2, are not working at the same time. Your computer (more specifically your operating system) is performing small calculations for each program one by one, switching between them so quickly the action looks fluid.
These operations are timed by the oscillation of the quartz. Because quartz can oscillate to such a precise rhythm, it can be used to manage the tiny intervals required for computer processing. Operations are performed according to beats of the system clock derived from the movement of the quartz. Imagine a child jumping rope: hop to let the rope pass underneath, rest otherwise. One operation on music, one on gaming, one on the mouse pointer, one on the keyboard, and so on. Your computer prioritizes multiple tasks with a hierarchy of interrupt requests — another node for another day.
Like I said before, the quartz on your motherboard is cut to a precise shape. The ways in which a clear crystal oscillates can be determined mathematically by anyone with a basic understanding of geometry.
Today, most industrial quartz is man-made and crystal-clear. Pressure along an axis results in electrical charge along an axis perpendicular to that of the pressure. Imagine a cross of pressure and electricity. In some Piezoeletric materials (like quartz), the principle can be applied conversely — direct electric current in a line, resonance flows outward at a right angle to the axis of electricity. Direct electricity into a perfectly cut crystal, place traces appropriately, you have a system timer.
Postscript: system timers do not violate the second law of thermodynamics.
Crystals don't oscillate unless their circuit includes an amplifier. Oscillation frequency also depends on the size and cut of the crystal — for example, there are 20MHz, 30MHz, 50MHz, etc crystals. Of course, these crystals resonate at higher frequencies, correct multiples of these base values.
Thanks go to IWhoSawTheFace.
Sources
Andrews, Jean. A+ Guide to Managing and Maintaining your PC. Thomson: Boston. 2004.
Hackman's Realm
http://www.geocities.com/SiliconValley/2072/xtals.htm
Fox Electronics Technical Theory of Operation
http://www.foxonline.com/techdata.htm