Let's look at the Core i5 750 CPU for a moment. Like almost all Intel CPUs in the Core i5/i7 line, the Core i5 750 has a BCLK of 133MHz. On the other hand, the rated speed of the i750 is 2.66GHz. The main processor takes the BCLK number and multiplies it by 20 to get 2.66GHz (2666MHz)--this is the CPU multiplier. Note that Intel's latest CPUs also have a feature called Turbo Boost that allows the CPU to run at clock speeds higher than the default speed under certain conditions. For example, when only one core on the Core i5 750 is in use, the Turbo Boost frequency is 3.2GHz.
Most retail processors are clock-locked, which means that you can't increase the CPU multiplier beyond its rated speed. Some motherboards try to cheat in an effort to unlock the CPU multiplier, but in most instances involving typical retail CPUs, you can't unlock the multiplier. Admittedly, you can usually set the multiplier to a lower number than the maximum rating, but it's unclear why you'd want to do this.
You can usually adjust the setting for an Intel CPU's base clock (BCLK) or for an AMD processor's CPU bus frequency to any value you want. As with all stages of overclocking, though, you have to be careful here: Changing the underlying fundamental clock frequency will change a host of other parameters. Still, it's a useful tool to help with overclocking.
What to Expect From Your Overclocked Processor
Before starting the physical process of overclocking, think about what you're trying to accomplish. If you use your computer to run standard desktop applications--office productivity apps, Web browsers, and so on--overclocking is not worthwhile, since the higher clock speeds won't deliver noticeably better performance.
On the other hand, if you run system-intensive applications such as games that hit all of the different subsystems in your PC--hard drive, graphics, memory, and CPU--you'll see some gain by juicing up the CPU clock, but don't expect too much. Often, even high-end games are not CPU bound, and they may benefit more from a better graphics subsystem than from overclocking. Still, you'll see some increase after adjusting the clock speed.
CPU-intensive apps--particularly multithreaded applications--are likely to see the most significant boost from increased clock speed. Photo editing and video transcoding are examples of these types of programs.
Again, however, remember that the ultimate goal is speed with stability. Extreme clock speeds are merely academic exercises if the overclocked system can't run your applications reliably.