A temperature sensor basically senses temperature, but it does it in a number of ways. A contact temperature sensor will read the temperature of an object that it is attached to physically. A non-contact temperature sensor can detect the temperature of an object that it is not attached to. This means that it can measure temperature of an object that is far away and does not have to touch the object at all.
Different Types of Contact Sensors
Contact sensors work in many ways. Thermocouples work using the Seebeck effect, which has to do with the temperature change in electrical circuits. Basically, the temperature is read from these sensors by taking measurements of their voltage outputs. Thermistors are another kind of common temperature sensor used in appliances, human thermometers and other objects. They have a predictable resistance that is affected by temperature change, so as the temperature increases or decreases the electrical current changes, or the resistance increases and decreases. A formula is used to interpret the temperature change. Since the change is reproducible these sensors can be very accurate.
Some of the more interesting temperature sensors never touch the object at all. They can be several feet or several miles away. You may have wondered how they measured the temperature of the sun. It was not with a giant thermometer on a pole. It was with a non-contact temperature sensor. These all use use Plank's Law to measure temperature, which deals with the amount of thermal radiation released by a heat source. By measuring the thermal radiation from an object, a temperature can be determined.