RTD stands for resistance temperature detector, which means that it works on the principle of variation in resistance with temperature. A thermocouple works on the principle that when two different metals are joined together, there is a potential difference at the point of contact that varies with changes in temperature

Difference between RTD and Thermocouple is that in RTD the resistance in ohms varies as per the temperature and normally used to sense lower temperatures up to 100 deg C whereas thermocouple generates a mV signal which varies with the temperature and thermocouples are normally used for measuring higher temperatures like 1000 deg C and above.