Air Conditioning units are all based on the same concept: removing humidity from the air. The difference is how efficiently the process can be done. SEER (Seasonal Energy Efficiency Ratio) addresses this issue, as it can be very confusing to a consumer.
The best way to explain this issue is to relate it to an every day process. If you knew your family had a lot of errands to perform every day and there was only so much money allowed in your gas budget would you drive a Suburban or a Corolla? The Corolla would get you there and also keep you on budget. The air conditioning SEER is the same concept. The higher the SEER the better the mileage, or the less you pay in fuel cost.
The unit itself must be designed with more intricate parts in order to operate efficiently enough to raise the SEER rating which in turn means the unit initially will cost more, but will save money in the long run. This is research the consumer needs to consider before purchase in order to consider the payback. To learn more, you can visit The Air Conditioning and Refrigeration Institute.