Thermistors are used in science and engineering applications. They are also useful in medicine as clinical temperature sensors or as probes during surgery. There are two types: PTC (Positive Temperature Coefficient of Resistance) and NTC (Negative Temperature Coefficient of Resistance).
NTC thermistors have temperatures that vary inversely with resistance such that as the temperature increases, the resistance decreases, and vice versa. They are very often used for temperature control and indication, and for current suppression. Common materials used in their construction include oxides of materials such as nickel, manganese, copper, iron, and cobalt. Some are also made from silicon and/or germanium. They are usually packaged in an epoxy, and are the most common type of thermistor.
PTC thermistors are the opposite of NTCs in that they have a resistance that increases with rising temperature and decreases with falling temperature. They are used to protect circuits from overload, and can function as thermal switches or as ordinary thermometers. PTCs are constructed using semiconductors combined with ceramics or polymers.
More information on their applications may be found at the AMWEI Thermistor Company. To learn more on PTCs vs NTCs, read this EE Times article.