A thermopile is an array of thermocouples which are connected in series. Thus, the strength of the voltage signal is increased. 
Thermopiles are used along with infrared sensors. A black body absorbs the infrared radiation from the source to be measured. This heats up the thermopiles which produces an output in millivolts.
Thermopiles are used in medical applications for measuring the body temperature
Thermopiles are also used in industrial measurement applications where they are fitted on to a suface, such as that of a bearing. The temperature of the surface can be measured.