What is difference between airpreheater and economiser

Air pre hetre is supply the heat to air and in ecomomiser heat absorb by the feed water.

An economizer system simply opens/closes a set of shutters to bring in cooler outside air when the outside ambient temperature is below the thermostat set point. It's kind of like free air conditioning :D.

Air pre hetre is supply the heat to air which is goiing to furnace for the combustion of fuel,,
Economiser utilises the heat from hot flue gases(exhaust gases) to heat the temperature of feed water,,