A vacuum flask is basically one flask inside another. The space between the walls has much of the air removed, which creates an empty space (or vacuum). The result of this is a dramatically decreased rate of heat transfer between the flask walls (in both directions), as there are fewer molecules available to move the heat from one wall to another (whether inwardly or outwardly). An additional feature is that the walls are silvered, or reflective; this further reduces the heat that is transferred by reflecting incident heat away from the flask wall again.