the "front side bus" or FSB is an alternate blanket term for "cpu frequency," "north bridge frequency," and "ram frequency." In short it's an overarching frequency for the northbridge chipset of your computer. It existed in a similar fashion on all AMD and Intel chips up until Sandy Bridge, when intel moved most of their northbridge and part of their southbridge onto the cpu itself. From that point on Intel stopped using FSB, and started using BCLK, which operated similarly to FSB, only it also included the frequency the PCI-E slots, USB ports AND SATA ports ran on as well, which meant overclocking through bumping the BCLK tends to be both dangerous and extremely limited. (AMD actually never used a FSB, though on motherboards it's often refered to as such, when AMD started to use hypertransport (HT) even their FSB version called EV6 vanished. That said, AMD continued to support a "cpu frequency" function, and on some motherboards its still called FSB)
If you bump the FSB you tend to be overclocking several things at once (northbridge, ram and cpu)... including ALL of the cpu cores. Now most motherboards are able to automatically adjust the multipliers on the northbridge and ram to make a FSB overclock invisible to you, and allow you minuet control over those parts of your system. That said older boards had no such features, meaning you had to manually adjust the multipliers on your ram and northbridge to find the speeds you wanted when you overclocked using the FSB.