When purchasing an air conditioner, furnace or heat pump, always ask about its efficiency rating.  This rating will tell you how efficiently the unit uses fuel (gas, electricity or oil).  The most common efficiency ratings are:

SEER (Seasonal Energy Efficiency Ratio): This ratio tells you the amount of cooling your system will deliver per dollar spent on electricity.  The SEER rating of any unit can range from 13 to 17.  The higher the SEER rating, the more efficient the system will be and the less it will cost in the long run to own and operate. 
HSPF (Heating Season Performance Factor):  Similar to SEER, it is a measurement of efficiency of the heating portion of a heat pump.  HSPF ratings range from 6.8 to 10; high efficiency units have efficiencies of 7.5 HSPF or higher.
AFUE (Annual Fuel Utilization Efficiency Ratio): A measurement of the percent of heat produced by a furnace for every dollar of fuel consumed.  The higher the AFUE rating, the lower the fuel costs.  All furnaces manufactured today must meet at least 78%.  Older furnaces (10-15 years or older) may fall below this minimum.  Furnaces with AFUE rating from 78% to 80% are considered mid-efficiency; units with an AFUE rating of 90% or above are considered high-efficiency.