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NTC and PTC thermistors, thermostats and thermal cut-outs,
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ATC Semitec Products
Ceramic PTC thermistors can be used as re-settable fuses. Over-voltage and over-current conditions can cause the PTC to "switch" from a low resistance into a high resistance state. This limits the through current and thus protects the circuit or load in series with the PTC thermistor.
270V PTC Fuse Range - 
PDF document
By varying the chemical composition of its ceramic material, the Curie point of a PTC thermistor can be altered to create a range of "trip" temperatures from 40oC to 190oC.
Basically the PTC's resistance increases dramatically after it reaches its Curie point and this increase can readily be used to "trip" an electronic circuit.
The PTCs are available in a radial format or can be supplied with flexible leads for electric-motor protection or in ring terminals to help monitor the temperature of heat-sinks or power components.
PTC Motor Protection range - PDF document
PTC Thermal Sensor Range - PDF document
By applying a voltage across a PTC thermistor, current will flow and begin to heat the PTC. The PTC element rises in temperature until it reaches its Curie point, when its resistance will start to rise dramatically. The increase in PTC resistance limits the through current until the PTC heater reaches a state of equilibrium. If heat is withdrawn from the PTC, its resistance will drop allowing more current to flow through the PTC until equilibrium is again reached. Conversely, if the PTC element temperature rises, its resistance will increase reducing the through current until a state of equilibrium is again achieved.
Thus a PTC thermistor can operate as a self-regulating heater.
Circular Disc PTC Elements - PDF document
Rectangular PTC Elements - PDF document