What do all those rating numbers mean?

The federal government requires all air conditioning and heating equipment to be rated for efficiency. The higher the rating, the more efficient the model.
 
Gas furnaces are rated for AFUE (Annual Fuel Utilization Efficiency). This defines the amount of heat used to warm your home from the burned fuel. A 90% furnace will use 90% of the available heat to heat your home. 10% of the heat is vented outdoors. Many older gas furnaces are only 60% efficient. The other 40% is vented outdoors. You can see that anew high-efficiency furnace will require much less fuel to heat your home.
 
For air conditioning, the rating is SEER (Seasonal Energy Efficiency Ratio). This rating is based on general design criteria such as the compressor and outdoor coil. Seer ratings are for comparison purposes only, so that homeowners will know how they can compare different brands of products with similar efficiency ratings. The rating for the outdoor unit will reference a general efficiency range, such as 14 SEER. The actual efficiency rating for a specific system will depend on the combination of the outdoor unit and the indoor coil. A variable-speed indoor blower unit will increase the efficiency rating of the system, as well.
 

Heat pumps are rated by SEER for cooling efficiency and by HSPF (Heating Seasonal Performance Factor) for heating efficiency. As with other ratings, the higher the HSPF, the less energy it will take to warm your home.