Thermistors are resistors which changes according to its temperature. When the resistance decreases, it is due to the increase in temperature. A 100k NTC thermistor has a resistance of 100kΩ (100 000Ω) at room temperature (20°C) and drop as low as 100Ω at 300°C.
There are various kinds of sensors to determine the temperature in the hot end. They are electronic thermometers. A thermistor is a common type of sensor but most of the time some printers use thermocouples for extremely high temperature printing.