In general, energy efficiency is defined as the "useful work" or "energy delivered" divided by the amount of energy supplied to do that work. With heat pumps, energy efficiency is measured in two different ways.
Heat pump efficiency is expressed as a Coefficient of Performance (COP). The higher the COP, the more efficient the system.
For example, a residential-sized geothermal system might have a COP of 3.4 or higher, meaning for every one unit of energy used to power the system, more than three units are put back into the home as heat. This compares to efficiencies of 0.92 for a high-efficiency natural gas furnace.
Cooling efficiency is measured as an Energy Efficiency Ratio (EER). The higher the EER, the more efficient the system.
Keep in mind that both COP and EER are dependent on many factors, and that high-efficiency equipment comes with a higher price tag - but the energy savings can pay back in the difference in just a few years.