http://en.wikipedia.org/wiki/Resistance_thermometer
http://en.wikipedia.org/wiki/Thermocouples

When should control engineers use a Thermocouple or RTD?

RTD
RTD = resistance temperature detectors
below 600 °C
High accuracy
Low drift
Wide operating range
Suitability for precision application

The most common of RTD is PT 100, R = 100 ohm at 0°C, sensitivity 0.385 ohm/°C

THERMOCOUPLE
A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference
range −180 to 2,320 °C