First is the Btu input, which rates overall capacity. Second is the Btu output, which rates usable heat delivered. The difference, for example, in a 100,000 Btu input furnace with an 80,000 Btu output, is the unit's combustion efficiency of 80 percent. You also can comparison shop using the EnergyGuide label, which rates appliances by energy use and predicted operating costs. On an old furnace, you can guess at the efficiency based on Btu ratings and how well it has been maintained, or by having it tested.
Some old furnaces are very inefficient and waste almost half the energy they use.
Gas furnaces manufactured since 1992, however, were required by the National Appliance Energy Conservation Act to have a minimum AFUE rating of 78 percent. Many newer furnaces are rated higher partly because of awareness programs such as the Department of Energy's Energy Star program. These days, the DOE's Energy Star program approves dozens of furnaces rated more than 90 percent and some up to 97 percent efficiency. Utilities also have a stake in efficiency because it saves them from constructing new power plants. That's what the rebates on high-efficiency equipment are all about.