Stands for "frontside bus." The FSB connects the computer's processor to the system memory (RAM) and other components on the motherboard. These components include the system chipset, AGP card, PCI devices, and other peripherals. Because the FSB serves as the main path from the processor to the rest of the motherboard, it is also called the "system bus."
The speed of the frontside bus is measured in Megahertz or Gigahertz, just like the processor. Most computers' processors run faster than their system buses, so the FSB speed is typically a ratio of the processor speed. For example, a Pentium 4 processor that runs at 2.4 GHz may have an FSB speed of only 400 MHz. The CPU to FSB ratio would be 6:1. A Power Mac G5, however, with a 2.0 GHz processor, has a 1.0 GHz frontside bus. Therefore, its CPU to FSB ratio is 2:1.
The smaller the ratio, the more efficiently the processor can work. Therefore, faster frontside bus speeds lead to faster overall performance. When the CPU to FSB ratio is high, the processor often has to wait for data to be sent out over the system bus before getting new data to process. For this reason, the FSB can be a bottleneck in a computer's performance. So if you are looking for a fast computer, don't just check the processor speed, but find out what the frontside bus speed is as well.