A thermistor (thermal resistor) - a resistor that changes its resistance with temperature. (NTC 10D-7)

They are popular for basic temperature feedback control.For example, if  you want to have a fan that turns on when the temperature gets high. You could use the thermistor to feed the base of a transistor, as the temperature rises, the resistance goes down, feeding more current into the transistor until it turns on. 

For thermistor data sheet,  please click here

For Using thermistor  with arduino tutorial,    please click here