SEER = Seasonal Energy Efficiency Ratio. SEER is used for central air conditioners and heat pumps and determines expected energy efficiency for the whole season, so the efficiency of the AC is measured at air temperatures from 65F to 104F. Because SEER is measured at various temperatures, it accounts for the AC starting and stopping.
EER = Energy Efficiency Ratio. EER is used for window air conditioners and determines energy efficiency at a single air temperature, 95F. Because EER is measured at a high temperature, it accounts for the AC running all the time (steady state).
Both are like gas mileage! Miles per gallon is replaced by the amount of cooling per watt hours of energy (fuel) used by the air conditioner. In SEER and EER, the higher the number, the more efficiently the unit uses electricity. SEER numbers are higher than EER numbers for any given model of air conditioner. A 5 ton 21 SEER unit can save you about $450/ year on your energy bill in comparison with a 5 ton 14 SEER unit.