A processor's operating frequency is reached by multiplying the effective Frontside Bus Frequency (FSB) and the CPU's multiplier.  As a simple example, a CPU multiplier set to x5 on an effective FSB of 100MHz would yield a 500MHz operating frequency.  Multiplier and recommended FSB settings are set by the manufacturers for different processor classes/speeds.  Running your processor at a frequency above the manufacturer specified rating for the device is called overclocking and, alternatively, running at a frequency below the specification is called underclocking.

This article is specifically written in reference to processor overclocking, however, it is important to note that one could theoretically overclock any device that is time dependent (your video card, memory, etc.)

So How Do You Overclock?

From the simple equation stated above:

Operating Frequency (Hz) = Effective Frontside Bus (Hz) x CPU Multiplier

we see that there are two degrees of freedom in controlling the operating clock speed of the system.  To force the overclock, one or both of these values can be altered to obtain the desired output.  Drawing again on the example above, if we were to set the CPU multiplier on the chip to a x6 value our operating frequency would jump to 600 MHz.  To obtain the same operating frequency one could also have left the multiplier at x5 and increased the effective Frontside Bus value to 120 MHz - once again yielding the 600MHz output.  What method you choose to overclock largely depends on whether or not your system presents you with the capabilities of altering settings and if your system is stable after you have implemented the changes.  It is important to note that some changes will require voltage alterations (usually more power is required to run a processor at the incorrect frequency) so stepping those values may also be an essential part of the process.

Earlier issue motherboards offered complete control over all of these settings through manual switches and jumpers that were to be adjusted during installation.  In such a configuration, one could simply set the series of FSB, Multiplier, and CPU voltage jumpers to higher than specified values in order to overclock their system.  With the movement toward automation, however, motherboard manufacturers would make things easier by incorporating these settings into software-driven setup programs and the casual overclocking industry took off. With the ability to make changes now so easy, anybody could overclock a system with very little effort.

The major processor manufacturers were troubled by this, however, as overclocking discouraged upgrades, greatly increased the chances of failure, and shortened the lifetime of their processors.  Measures were taken by them to give less and less control over the operating frequency settings and to discourage people from altering them.  One of the first of these measures was to slowly control and ultimately freeze the multiplier settings on the CPU.  Multiplier settings are now hard-set on the CPU and usually can only be altered (if at all) through a process of connecting traces on the CPU surface.  This not only is a delicate physical process, but also a difficult one as you must have knowledge of the CPU hardware, have access to it (ie- if it is installed already you must uninstall it and pull it from your system), and be willing to risk damaging it through an improper setting.  Most people also shy away from physically altering their processor as doing so will void any warranty.

With the multiplier problem largely controlled, the CPU manufacturers received a little help from their motherboard partners in controlling the FSB settings.  Many systemboard manufacturers decided to put in limitations on what FSB values their products would support - oftentimes opting to make steep jumps between settings.  The reason this hurt overclockers was that they no longer could trial-and-error the frequency values in small increments (hoping to push to the maximum value without creating a system instability).  By making the jumps very steep, it usually became too much of a push between two values and the overclocker would have to revert back to a lower rate for stability.  This, however, was still better than the automatic approach other manufactures took - resetting the FSB to the exact specification on the CPU on each reboot.  All in all, overclocking became much more difficult...but not impossible.

Some motherboard manufacturers have taken the opposite route and do cater to the overclocker community.  This has proved to be a very successful marketing tactic as overclockers tend to be people who care for and are willing to spend more money on their systems.  If you are looking to venture into overclocking it is important to read up about the motherboard / CPU combination you will be using to see what results others were able to attain.  Expect to pay more for a motherboard that gives you more control and has better stability results.

What Do You Need?

Besides the overclockable motherboard and CPU combination, the greatest concern to an overclocker is heat control.  A great deal of money is spent by AMD and Intel in the process of rating a CPU for commercial release.  Much of this is spent to achieve acceptable heat output values at the designated frequency of the device.  By pushing past their specification, you can expect your processor to output quite a bit more heat than it was intended to.  Failure due to overheating is the single biggest problem with overclocking and can lead to damage throughout your system so it must be controlled.  Depending on your application and how far you want to overclock, this may mean that you will need to upgrade your CPU Fan / Heatsink or install an alternative cooling solution to account for the difference.

There are many sites and resellers on the internet dedicated to dealing with the heat issue, but not all methods will come cheap.  For the more involved overclocker, alternative cooling systems such as Peltier based or watercooling units may be the best bet, but can cost anywhere from $75-200 or more.  With this, these systems can be very noisy and can be very difficult to install.  With watercooling you also introduce condensation issues which can be very tricky to control.

What Can Go Wrong?

Overclocking can be a very dangerous practice and you should always act with caution and proceed slowly.  Jumping to a value you are unsure about could spell doom in the long run.  Oftentimes the safest approach is to take every step slowly and to test for stability before moving to a higher level.  Also, make sure that you have an adequate cooling solution that will exceed the added heat output you expect to create from the overclocking.  It is very easy to burn-out of 'fry' your processor as a result of not controlling this heat so it can be quite costly.  Overclocking can also reduce the life of a CPU, create system instability issues, and introduce added risks while in operation.  Undoubtedly, all manufacturers have banned the practice so by overclocking your chip you are voiding your warranty.  With all of this said, however, if you are able to achieve noticeable differences in performance values it just may still be worth it.