There are a few ways to measure temperature electronically. You mentioned a thermistor, so I will start there:

The thermistor is a device where the resistance of a metal changes with the temperature. Some materials increase resistance when the temperature increases (positive temperature coefficient, PTC), and others decrease resistance with an increase in temperature (negative temperature coefficient, NTC). PTC thermistors are also called RTD, or Resistance Temperature Detectors. The temperature is found by knowing the metal, and measuring the resistance.  

There is also a thermocouple. When two dissimilar metals meet, there is a specific voltage created at the junction. With certain metal combination, this voltage is effected by the temperature. So by measuring this voltage, and knowing the materials used in the thermocouple, you can tell the temperature.

The device used depends on the accuracy required, as well as the operating range of temperatures.

All of these devices can have flexible wires attached. The metals used depend on the type of temperature sensor you have (PTC/RTD's typically use platinum). These devices should be insulated as they all tell the temperature by measuring the electrical properties (resistance or voltage) across the closed circuit.