Demystifying how today’s high-efficiency furnaces make it all happen
Your gas furnace is made up of the following major components: a heat exchanger, a blower (or fan) and a burner.  So let’s start with the air flow, your blower accomplishes two jobs at once, 1) it pulls in the air from your home into the furnace itself and 2) it blows that air up through the furnace to get heated and then back into your home.
The heating part of the process starts with the ignition of the gas that is released through the burner which is usually situated at the top of the furnace’s heat exchanger.  Today’s modern furnaces do not have pilot lights but rather electronically ignite the gas as it is released by the burner valves.  Furthermore, they pull in either fresh air from outside or ambient air from around the furnace itself to aid with more complete combustion.
Now that the burner is lit, a small fan situated at the bottom of the heat exchanger pulls this heated gas down through it.  This is a good place to give a quick explanation of what exactly a heat exchanger is.
In essence, the heat exchanger is a chamber around which the cooler air from your home mixes with and absorbs the heat created from the hot gas produced at the burner.  In high efficiency systems the key is to keep this hot gas in the exchanger as long as possible so that the maximum amount of heat can be transferred over to the cooler air.  This is achieved by creating bends and curves in tubing through which the heated gas passes through to both create a longer distance to travel and to slow the rate at which it passes over this distance.  This is also why the burner is usually located at the top of the heat exchanger and has a small fan at the bottom of it pulling it down through the exchanger.  This way we can fight gravity and pull the gas downward which helps keep it in the chamber longer.  The cooler air from your home is then passed over these now hot tubes allowing for the heat transfer.
The highest efficiency furnaces (condensing furnaces) actually have a secondary heat exchanger that further compresses the hot gas to the point of condensing the water vapour contained in it into liquid water.  This chemical change of state releases heat energy in the process over-and-above the heat generated from the burners alone.