RTD's are stable and have a fairly wide temperature range, but are not as rugged and inexpensive as thermocouples. Since they require the use of electric current to make measurements, RTD's are subject to inaccuracies from self-heating.
An RTD capitalizes on the fact that the electrical resistance of a material changes as its temperature changes. RTD's rely on the resistance change in a metal. The resistance will rise more or less linearly with temperature.
Traditionally, RTD's use a length of conductor (platinum, nickel iron or copper) wound around an insulator. Newer styles use a thin film of the conductor deposited on a ceramic substrate.
RTD's are used to measure temperatures from -196° to 482° C (-320° to 900° F)