Phyzzle said:
Actaully, SI is another name for the metric system.
I think you mean to contrast SI/metric units with customary/imperial units.
Actually, unless my memory mistakes me, SI is an international standard of measurements used in science and engineering. Depending on what you are measuring (i.e., rotational force, energy, volume, work, momentum, temperature, etc...) you use a certain unit of measurment (i.e. joules, grams, newtons, kelvins, cm, etc...) not exclusively metric. It just so happens most SI units are metric.
Though I will admit, I could be wrong.