Heat travels from a warm area to a cool area by a combination of conduction, convection, and radiation. Heat flows by conduction from a hotter location within a material or assembly to a colder location, like the way a spoon placed in a hot cup of coffee conducts heat through its handle to your hand. Heat transfer by convection occurs when a liquid or gas- air, for example- is heat, becomes less dense, and rises. As the liquid or gas cools, it becomes denser and falls. Radiant heat travels in a straight line away from any surface and heats anything solid that absorbs its energy.
Heat flows from warmer to cooler until there is no longer a temperature difference. In your home, this means that in winter, heat flows directly from all heated living spaces to adjacent unheated attics, garages, basements, and even to the outdoors. Heat flow can also move indirectly through interior ceilings, walls, and floors- wherever there is a difference in temperature. During the cooling season, heat flows from the outdoors to the interior of a house.
To maintain comfort, the heat lost in the winter must be replaced by your heating system and the heat grained in the summer must be removed by your cooling system. Properly insulating your home will decrease this heat flow by providing an effective resistance to the flow of heat.