The key components of a refrigeration system include the compressor, condenser, expansion valve, and evaporator. These components work together to transfer heat from one location to another, allowing for cooling or heating of a space. The compressor compresses refrigerant, which then flows to the condenser where heat is rejected. The expansion valve reduces the pressure of the refrigerant, allowing it to expand and cool, and the evaporator is where the heat is absorbed from the surrounding environment.