Forced air systems move air by the use of a fan (and sometimes other mechanisms).

These systems are easy to recognize because they contain a network of ducts that expel air through a diffuser and distribute heated or cooled air throughout the building.

The air in a forced air system is typically heated by a furnace or heat pump, and cooled by an air conditioner.

It is fair to say that most commercial buildings used forced air systems. Many and possibly most residences used forced air as well.