a temperature dependent resistor made of a semi-conductor, thermal energy frees up electrons and increasing temperature decreases resistance.
practical uses of thermistors

thermostats control heating systems and there is a thermistor in the thermostat, if resistance falls then temperature goes up, if it rises then the temperature goes down.
the resistance of LED decreases if light intensity increases.
thermistors are useful temperature sensors, car engine sensors or electronic thermostats
in hot conditions the resistance drops, in cold conditions the resistance goes up
