FSB is an abbreviation for Front Side Bus; it is the internal data channel connecting a computer’s processor (CPU), chipset, RAM (all flavors), motherboard busses and AGP socket. FSB is described in terms of its width in bits and it’s speed in Mhz. In everyday terms, it is the doorway for the CPU to talk to the system bus, and how fast the bus can talk to other computer components.
In architectures where the processor interacts directly with main memory, the definition of a singular front side bus is less clear. In such a case you would generally specify two FSB speeds, one for the connection to main memory and one for the connection to the processor chipset.