Thermistor are resistor whose resistance changes with temperature. When the temperature increase, the resistance decrease. A 100k NTC thermistor has a resistance of 100kΩ (100 000Ω) at room temperature (20oC) and drop as low as 100Ω at 300oC.
These are all various types of sensors for determining the temperature of the hot end. They are essentially electronic thermometers. Thermistors are the most common type of sensor, but some printers will use thermocouples for extremely high temperature printing.