To expand a bit:
In generic terms, a "bus" is a set of wires that connect the computer's processor to anything else. Your computer has a "memory bus," that connects the processor to RAM; a "front side bus," that connects the processor to the cache; a "PCI bus," that connects the computer's processor to PCI cards; and so on.
Depending on the particular model of computer, some of these busses may or may not be overclockable. For example, on many PCI Power macs, the clock that runs the processor also drives the memory bus; it is possible, on some models of computer (like a Blue and White G3), to increase the speed of both the processor and the memory bus.
Most upgrade cards increase the processor's speed only. Some upgrade cards also speed up the memory bus. Speeding up the bus is a dangerous thing to do, because the RAM may not be fast enough to keep up.