Heat Loss: A calculation which determines the maximum amount of heat (Btu/h or KW) that is required to condition a building based on the outdoor winter design temperature.  A Heat Loss considers thermal heat transfer, air leakage and mechanical ventilation in its calculation.  

Heat Gain: A calculation which determines the optimum amount of cooling (Btu/h or Tons) that is required to best manage temperature (sensible) and humidity (latent) of a building.  A Heat Gain considers thermal heat transfer through the assembly including solar gains through windows, air leakage and internal heat gains from people and appliances in its calculation.