The term downflow is used to describe the airflow of a given furnace or air handler. The term is only used to describe the airflow of the indoor system. In a downflow furnace application, the air conditioner evaporator coil will sit below the furnace. In most applications, the furnace air filter will reside above the furnace, and the evaporator coil will sit near the ground or floor.
Most downflow systems will be evident if your home is on a slab or crawl space, but you still have floor registers. If you have a basement system, it is more likely you have an upflow system.