RTDs and Thermistors are both electrical devices designed to measure temperature. The main difference in the two is the materials they are made from, Thermistors are usually made with ceramic or polymer materials, whilst RTDs are made of pure metals.
RTDs can read temperatures up to 600°C but are generally used much lower to take advantage of the high accuracy. Thermistors can read temperatures up to 200°C and have faster response times than RTDs.