Thermistor and thermocouple both can be used to measure temperature. Basic difference between thermistor and thermocouple is that “A thermistor can only be used at fairly moderate temperatures. A thermocouple is used to measure typically high temperatures.”
A thermistor is a type of variable resistor that is temperature sensitive.
How does thermistor work?
When its temperature coefficient is negative, the resistance changes inversely with temperature. When its temperature coefficient is positive the resistance changes directly with temperature.
The resistance of photo-conductive cells changes with a change in light intensity. This cell also has a negative temperature coefficient. Symbols for both of these devices are shown in the below figure.