What is the difference between a 1-stage and a 2-stage heat pump?

1 stage heat pump compressors run at one single speed, which can produce too much cooling, the output is regulated by switching the compressor on and off, which causes a lot of mechanical stress on both motor and compressor especially in the shoulder months when not a lot of cooling/heating is needed. 2 stage has a 2 speed motor depending on the demand it either runs slower or fast, which gives better efficiency, more comfort, lower bills and longer lasting equipment. The Best choice today is an inverter driven scroll (rotary)compressor, now your heat-pump can continually change speeds to match the load, it will have soft starts and landings and is the ultimate in comfort and frugality.