A thermistor is a resistor-like electronic component that shows a large resistance change with a fairly small temperature change. This makes them useful in temperature measurement and control.
With the NTC (negative temperature coefficient) thermistors most commonly used, resistance falls as temperature rises.
The resistance at any known temperature can be forecast from the thermistor specification. Usually, suppliers quote the resistance at room temperature (25°C) and a Beta value (B).
Then resistance at temperature T°C is :
Where e = 2.7183 and temperatures are in Celsius (adding the 273 converts them to degrees Kelvin for the formula to work)
Thermistor Calculation Examples
RS Datasheet on their thermistor range (pdf 95k)