This refers to how the AC system takes in air and dictates the how each system can be installed.
Updraft/Upflow
Updraft, which may also be referred to as upflow, is an air cycle that pulls air from outside the furnace. Once the air is warmed inside the furnace, it is sent upward by the fan, creating an updraft of air. This air flows into ductwork that spreads it through the house until the thermostat senses the correct air temperature has been reached.
Downdraft/Downflow
Downdraft units, which can also be called downflow, are air conditioning systems that pull in air and deliver it downward once it has been heated. In this case, the fan directs air down below the furnace unit into a duct system. Once there, the air is spread normally through the house.
Location
The primary difference between an updraft and downdraft unit is where you can install them. The updraft unit is common and works best if you can place the system in your garage or basement. If you already haven an updraft unit, you will probably need to replace it with the same type. A downdraft unit is used when there is no basement space, but the house has enough attic space to accommodate the system.
Savings and Efficiency
There is no significant difference between updraft and downdraft units when it comes to saving energy and utility costs. Fans may need to work less on downdraft units, but the inclination of air to rise and the effects of gravity may cancel each other out. Saving money depends on the efficiency rating of the system and getting the appropriate size system for your house.