1. What is the difference between air conditioner and Refrigerator - by cycle,by parts. Can refrigerator be used as a air conditioner. If not why?
2. How humidity is controlled in air conditioner.
3. Which part does the work of humidity control in AC and how - compared to that in Refrigerator.

I believe that they cool air in the same way, using Freon in a cycle to take the heat from one area and moving it to any other by way of a heat pump.

Air conditioners also control humidity and other factors in order to 'condition' the air to be comfortable for humans. Fridges don't do this.

A fridge would provide a similar cooling effect to an air conditioner if you stuck the back of it out a window. However I doubt it would be as strong as it isn't designed to cool large volumes of air

Compare
http://en.wikipedia.org/wiki/Air_conditioner
And
http://en.wikipedia.org/wiki/Refrigerator

For further information