FSB ~ rpms on a cars motor.
Multi ~ gear box.
CPU speed ~ The speed attained from the previous 2 combinations.
Many CPU's have a various multipliers that can be used (i.e. 7-9).
Using the multiplier and the FSB in various combinations will yield various performance outcomes.
If you are new to OC'ing, you may want to lock your RAM at it's stock speed (sometimes this is not possible [i.e. P5B-D could only be advanced ~ at least while I was using it and with the BIOS version I was using]).
Voltage to the core will also play a part... as will the thermal output from the CPU's calculations. Please know your thermal limits, this will save you from some distress.
note: That is a tuncated and glib interpretation, but the analogies helped me a lot.