What is the difference between "combustion heat" and "calorific value"?

The "gross calorific value" (also known as the "higher calorific
value) and the "heat of combustion" of a material are the same thing. 
Both represent the heat released when a unit mass or volume of a
material at 1 bar pressure and 25 C is completely combusted and the
combustion products are brought back to the starting pressure and
temperature.

You may also see the term "net calorific value" (also known as the
"lower calorific value" or "inferior calorific value".  This quantity
assumes that the water produced by combustion remains in the vapor
phase in the exhaust, and is lower than the gross calorific value by
the latent heat of condensation (joules/gram) of water at 25C
multiplied by the concentration of water in the material (expressed as
grams/gram of fuel).  For most common fuels, the net calorific value
is about 10% less than the gross calorific value.