Front Side Bus (FSB): This is the spead at which the motherboard communicates with the other components.
CPU External Frequency: This is the FSB speed that the CPU operates. This frequency is doubled when referring to the CPU. For example, the Athlon 3200+ is a 400mhz chip, which means it operates at 200mhz FSB. Any future mention of FSB is referring to this setting.
CPU Frequency Multiple: Commonly referred to as the "Multiplier," this number is multiplied by the FSB to equal the CPU Clock Speed. For example, A 133mhz FSB CPU with a multiplier of 15 runs at 1995mhz. (Multiplier * FSB = Clock Speed)
Memory Frequency: Specifies the FSB frequency of the memory. Like the CPU, the FSB is doubled when referring to memory - for example, DDR400/PC3200 Memory operates at 200fsb.
Hope this helps.