G'day,

A MOV works much like a zener diode. It is put in parallel with the input circuitry and presents a large impedance below its trigger voltage. Above this voltage, it shows a low impedance and will "sink" or pull the input voltage low, effectively "clamping" the voltage.
What value you use is dependant on the circuit, but the main considerations are the maximum tolerance of your circuit (ie max voltage it can withstand) and the time response required (ie what is the minimum response time ).
A lot of so called transient suppressors are based on MOV's but fail miserably when dealing with lightning strikes because the voltage surge has been and gone before the MOV has time to react.
Hope that helps,
Peter