The thermocouple wire or cable is used to manufacture thermocouple probes in order to sense temperature using the point of cold junction compensation (cjc end). On the other side, as it has been explain in this other article, a thermocouple is a sensor for measuring temperature, that consists of two dissimilar metals that are joined together at the sensing end.
Different thermocouple types (e.g. J or K type thermocouple wire) use different mixtures of metals in the cable. At the cjc end, the millivolt value provided by the thermocouple represents the difference in temperature of the sensing end as compared to the cjc end (also called the reference end)...
Thermocouples are the most widely used temperature sensors on the planet due to their combination of wide temperature capabilities, ruggedness, and low cost. Thermocouples are used in applications that range from home appliances to industrial processes, to electric power generation, to furnace monitoring and control, to food and beverage processing, to automotive sensors, to aircraft engines, to rockets, satellites and spacecraft.
When it comes to high temperatures, small size, fast response, high vibration or shock, most times you will find thermocouples providing the temperature measurements.