Upflow Furnace
An upflow furnace draws cold air in through the bottom and sends heated air out the top. Upflow furnaces are often used in houses that have basements or that deliver heat through overhead ductwork.
Downflow or counterflow furnace draws cool, return air through the top and delivers heated air out the bottom. This type is favored where there is no basement or where air ducts are located in the floor.
Source:http://www.hometips.com/hyhw/comfort/42upflow.html