A fan switch is a normally open switch, designed to close when a set temperature is reached.  These switches are commonly used in gas furnaces to control power to the blower motor.  Temperature listed is Fahrenheit.  

The standard size fan switch is about the size of a quarter.  The switch mounts with two screws on an oval mounting plate. Terminals are 1/4" spade style.  

Temperature differential refers to the temperature difference between the point that the contacts close and reopen.  For example if the fan switch is marked F120-10, that would indicate that the switch would close at 120º and open at 110º.

Standard cup sensor is approximately  1/2" deep

Deep cup sensor is approximately 1" deep

Fan switches are electrical devices that are not returnable.  Defective merchandise is replaced with an identical item.