A computer reduces every task to a series of calculations and the processor -- sometimes called the central processing unit -- physically carries out these calculations. In principle, the processor's clock speed, which is the measure of the speed of these calculations, determines how fast the processor performs. In practice, several other factors affect the performance of a processor and thus how fast it appears to work.
The clock speed is the most common way to measure a processor's speed. It's expressed as hertz, which is the number of times per second that the electrical voltage in the processor switches back and forth between low and high. The processor uses these oscillations to physically carry out the calculations -- low and high peaks correspond to the zeroes and ones that make up binary computer instructions. Most computers today have a processor that works at several gigahertz: one gigahertz is a million hertz.
Some processors are dual- or quad-core units. This arrangement means they are set up to work as if they were two or four separate processors. A dual- or quad-core computer can work more efficiently because it can assign one or more cores to each particular task but can leave other cores free for other tasks. This division means the computer can carry out a complicated task such as video editing while still being able to run other applications without delays. The clock speed refers to the processor as a whole: it runs at this speed regardless of how the cores are assigned to separate tasks.
Overclocking involves modifying a computer so that a processor runs at a faster clock speed than its default setting. This is a complicated task and you should not do it unless you are knowledgeable about computer hardware. Overclocking increases power consumption and elevates the risk of overheating that leads to processor damage.
Most processors use a cache, which stores data from the computer's memory that's ready for processing. The bigger the cache, the less time the processor needs to spend waiting to transfer data from the main memory. Caching doesn't increase the clock speed itself but it helps the processor to run at its full potential.
Some computers have a separate processor dedicated solely to graphics, freeing up the main processor for other tasks. A combination of a main processor with a slower clock speed and a graphics processor may make a computer run faster overall than a processor with a faster clock speed but no separate graphics processor.
The world's fastest computers usually have many processors working together. These machines carry out particularly intensive tasks such as running simulations of complex systems that have billions of different potential outcomes -- like modeling weather forecasts. Supercomputer speeds are usually expressed in flops, or floating point operations per second, rather than hertz. The fastest supercomputers can work at numerous petaflops per seconds: a petaflop is a million billion flops.
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