The Vikkster has it down, but you wont have to disassemble to decide which you've got.
As I have seen, original equipment IS the thermal clutch that has a bi-metal, coiled spring on the front of it. The centrifugal does not have the spring, and uses a viscous fluid coupling that allows it to 'freewheel' at engine speed. The thinking is that at higher speed you are getting the primary air flow from driving.
When replacing you most usually have an option and the thermostatic IS less expensive.