SEER,  Seasonal Energy Efficiency Ratio, is most commonly used to measure the efficiency of a central air conditioner. It measures how efficiently a cooling system will operate over an entire season. The higher the SEER, the more efficient the air conditioner. Technically speaking, SEER is the ratio of the cooling output in Btu divided by the power consumption in watts per hour.

Energy Efficiency Ratio (EER) is a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level (95^oF). In technical terms, EER is the steady-state rate of heat energy removal (i.e. cooling capacity) by the product measured in Btuh divided by the steady-state rate of energy input to the product measured in watts. This ratio is expressed in Btuh/watt. The higher the EER, the more efficient the air conditioner.

Both SEER and EER are included in the ENERGY STAR specification because each rating indicates the energy efficiency of the product under different operating modes. SEER rating more accurately reflects overall system efficiency on a seasonal basis and EER reflects the system’s energy efficiency at peak day operations. Both ratings are important when choosing a product.

Source  www.energystar.com