Stands for Front Side Bus, which denotes the speed at which your processor interacts with the components on the motherboard. Typically the FSB is 100Mhz or 133Mhz, but overclockers often manipulate this value to increase the speed at which their processor runs. i.e. 100Mhz FSB X 5.0 clockmultiplier = 500Mhz processor
Web Definition: FSB - Front Side Bus is the data channel connecting the processor, chipset, DRAM, and AGP socket. FSB is described in terms of its width in bits and it's speed in MHz.