PT100 sensors are high temperature, high accuracy temperature sensors and suitable for hotends which run up to 400°C. This high temperature is needed for more special materials like Nylon, Polycarbonate, Peek, etc. The name PT100 is derived from the material used in the sensor Platinum (PT) and the 100 ohm resistance at 0°C.
The PT100 sensor will not fit in the standard E3D Hotends, but can be used with E3D V6, Chimera, Lite6, and Kraken by using the PT100 Heater Block available in the spares and extras section.
Please be aware that a PT100 is not the same as a thermistor and unless your electronics explicitly supports RTD Sensing devices you will need an amplifier board like the E3D PT100 Amplifier board or the Duet PT100 daughter board.