As the PTC thermistor heats, its resistance increases rapidly, so reducing the heating effect. This is a self regulating heater, though the heat may only be small.
A resistor made of metal has a positive temperature coefficient like 0.39% per degree C for copper (platinum is similar). An RTD (also called a PRT when platinum wire is used) has this small small sensitivity, the same as copper connecting wires, but can be made in precision interchangeable types. The temperature versus resistance is highly predictable, reasonably linear, and covers a much wider range of temperature than a single thermistor. Circuitry needs to be carefully designed to reduce errors due to the low sensitivity. They are popular in industrial situations.
The temperature sensor you show is one with a protective metal case. The thermistor bead is inside.
The link below compares temperature vs resistance for 3 types of thermistor and an RTD.