HSPF = Heating Season Performance Factor. This is a term used to measure the efficiency of air source heat pumps. The efficiency is measured over an average heating season. HSPF is a ratio of BTU heat output to watt hours of electricity used. It has units of BTU/watt-hour. The higher the rating, the more efficient the unit.
SEER = Seasonal Energy Efficiency Ratio. This is a measure of a heat pump's efficiency over an average cooling season, using cooling output in BTUs compared to electricity consumed in watt-hours.
COP = Coefficient of Performance. This is the ratio of cooling or heating output to energy consumed at a specific temperature. This ratio converts both output and consumption to a common unit, making this different than HSPF or SEER which use a ratio of BTUs to watt-hours. The higher the COP, the better.