There are two basic ways of heating a home; radiant heat room by room, or a central heating system. Radiant in each room can be electric, wood stove, gas heater , kerosene, coal, a fireplace, etc. Central systems can be hydronic, steam or forced warm air. Hydronic and steam are detailed under hydronic elsewhere in this site.
A forced warm air system uses ductwork to distribute heated air from a source (furnace or air handler) to each room. The furnace can produce heat from any number of fuels; gas, oil, electricity, wood, or coal, or a combination of any fuels. An air handler will use a hot water coil to produce heat (see hydro-air).
Unless fresh air is piped in from outside of the home, the system will re-circulate 100% of the air it supplies. This means it must obtain air from the home by way of a return air duct or ducts. Properly installed, a warm air system becomes a loop by which air is drawn from the living space through return ducts to the furnace, heated, and sent back to the same space through supply ducts. The advantages to this type of heating system are numerous.
The air can be heated, cleaned, sterilized, humidified, or cooled (central air conditioning). If return air ducts are strategically located, the will reduce heat loss by recycling the warmest air back in to the system that collects at upper areas of the house (see return systems).
Supply ducts located around the outside walls of the rooms will temper the cold air as it infiltrates the home and reduce any discomfort from the air flow to a minimum.
The disadvantage is that ductwork takes up space. When installed by an experienced contractor, the ductwork will take up minimal or no extra space, and literally disappear into the framing of a house.
The heat loss, zoning, and cfm calculators in this site were designed for forced warm air systems, but will work for all residential applications.