Iron Furnace
An Iron Furnace works much like a traditional Furnace, except that it is 56.25% more efficient with the fuel you use.
You can see in the below recipes that both use the same amount of fuel, but the Iron Furnace produced an extra 9 ingots compared to the traditional Furnace.
For example:
16x Iron Ingot + 2x Coal will produce 16x 
Refined Iron Ingot in Furnace
25x Iron Ingot + 2x Coal will produce 25x Refined Iron Ingot in Iron Furnace