Both RTD and thermistor sensors use resistance in the sensing element to calculate the temperature. RTD and thermistors can achieve very good accuracies at the range that they cover while being simpler than other sensors, such as thermocouples, to integrate into a circuit.
RTD's and thermistors differ mainly in temperature range. Thermistors usually have a short range, typically up to 150°C whereas a typical RTD can reach up to 500°C. RTD's are also usually more stable thus making them better for long term use. Thermistors have the advantage in price and sensitivity, which makes them very popular in large quantity productions.