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Every modern PC has multiple system clocks. Each of these vibrates at a specific frequency, normally measured in MHz (megahertz, or millions of cycles per second). A clock "tick" is the smallest unit of time in which processing happens, and is sometimes called a cycle; some types of work can be done in one cycle while others require many. The ticking of these clocks is what drives the various circuits in the PC, and the faster they tick, the more performance you get from your machine (other things being equal).
The original PCs had a unified system clock; a single clock (running at a very low speed like 8 MHz) drove the processor, memory (there was no cache back then) and I/O bus. As PCs have advanced and different parts have gained in speed more than others, the need for multiple clocks has arisen. A typical modern PC now has either four or five different clocks, running at different (but related) speeds. When the "system clock" is referred to generically, it normally refers to the speed of the memory bus running on the motherboard (and not usually that of the processor).
The various clocks in the modern PC are created using a single clock generator circuit (on the motherboard) to generate the "main" system clock, and then various clock multiplier or divider circuits to create the other signals. The table below shows the typical arrangement of clocks in a 266 MHz Pentium II PC, and how they relate to each other:
The entire system is tied to the speed of the system clock. This is why increasing the system clock speed is usually more important than increasing the raw processor speed; the processor spends a great deal of time waiting on information from much slower devices, especially the system buses. While a faster processor will have greater performance, this increase in speed will not lead to nearly as much performance improvement if the processor is spending a great deal of time sitting idle waiting for other, slower parts of the system.