What do you mean by 'ocean thermal energy'? How can electricity be generated from this energy?

(N/A) Ocean thermal energy refers to the energy available due to the temperature difference between the water at the surface of the ocean and the water at deeper levels.
Due to sunlight,the water at the surface of the sea gets heated up,while the water in the deeper regions remains relatively cold.
This temperature difference is utilized to generate electricity.
The warm surface water is used to boil a volatile liquid like ammonia.
The high-pressure vapours of the liquid are then used to run the turbine of a generator,which produces electricity.