Thermistors are usually a thin coil of a semiconducting material. They work because raising the temperature of a semiconductor increases the number of electrons able to move about and carry charge - it promotes them into the conducting band. The more charge carriers available, the more current a material can carry. This is described in the formula:
i=nAve (Where i is current, n is the number of charge carriers, A is area of the material, v is voltage and e is the charge on an electron)
The current is measured using an ammeter. Over large changes in temperature, callibration is necessary. However, this is unnecessary if the right semiconductor is used, because over small changes in temperature the resistance of the material is linearly proportional to the temperature. There are many different semiconducting thermistors and their range goes from about 260°C to 1700°C.