Answer:
When the motion of the fluid results due to natural means like the buoyancy effect, that is, the elevation of the warmer fluid and the fall of the cooling fluid is termed as natural convection. Boiling water is the everyday illustration of the natural convection, in the process, the passing of the heat takes place from the burner into the pot, and heats the water present at the bottom.
On the other hand, when the fluid is forced to flow in a tube or over the surface by external means like with the help of a fan or a pump is termed as forced convection. The use of a fan on a hot summer day is an everyday illustration of forced convection. An example of effective heat transfer is the sweat produced by our body. Therefore, when one switches off the fan, the air present around the body captivates the water vapor until it gets saturated. It takes place for some time and stops eventually, which makes one feel hotter.
At that time, when the fan is switched back again, the air surrounding the body starts to move again, without completely getting saturated, and thus, evaporation of the sweat takes place that also contains the heat of the body and one feels cooler.
