The main parts of a thermos are the stopper, an inner wall, an outer wall and a base to keep the walls in place. The thermos was first patented in 1907.
Simply stated, a thermos stops the conduction of heat, either in or out. The vacuum created by the inner and outer walls is the key mechanism. If a cold beverage is put in a thermos, the vacuum stops the outer wall from transferring heat to the inner wall and warming the cold liquid. Conversely, but using the same method, the vacuum inhibits heat from a hot beverage in a thermos from transferring out of the container.