EER stands for Energy Efficiency Ratio. EER values are commonly used when looking at the energy efficiency of smaller window units. The EER will not give you an accurate representation of your future energy bills, but it can provide a good way to compare two systems.
Both SEER and EER measure energy efficiency. While SEER measures seasonal efficiency, the EER is more of a constant number. As opposed to SEER which is calculated over a range of outside temperatures, EER is typically determined by a set outside air temperature, a set inside air temperature and a 50% relative humidity. For a comparison point, 11 EER is roughly equivalent to 13 SEER. The EER is usually used for smaller window air conditioners. SEER is the more common energy efficiency metric for typical heating and cooling systems.