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Processor / Memory Data Bus
Every bus is composed of two distinct parts: the data bus and the address bus. The data bus is what most people refer to when talking about a bus; these are the lines that actually carry the data being transferred. The wider the data part of the bus, the more information that can be transmitted simultaneously. Wider data buses generally mean higher performance. The speed of the bus is dictated by the system clock speed and is the other main driver of bus performance.
The bandwidth of the data bus is how much information can flow through it, and is a function of the bus width (in bits) and its speed (in MHz). You can think of the data bus as a highway; its width is the number of lanes and its speed is how fast the cars are traveling. The bandwidth then is the amount of traffic the highway can carry in a given unit of time, which is a function of how many lanes there are and how fast the cars can drive in them.
Memory bus bandwidth is extremely important in modern PCs, because it is often a main bottleneck to system performance. With processors today running so much faster than other parts of the system, increasing the speed at which data can be fed to the processor from the "outside" usually has more of an impact on overall performance than speeding up the processor itself. This is why for example, a Pentium 150 is not much faster than a Pentium 133; the P150 runs on a 60 MHz memory bus and the P133 on a 66 MHz bus. 10% more clock speed on the system bus improves overall performance much more than a 10% faster processor.