Hair dryers work through two basic components that operate independently from one another: a heating device and a fan that takes in air from the side and propels it out the main barrel. Hair dryers function by increasing the temperature of the air around the hair, helping water to evaporate from the hair. The heat helps individual water molecules to separate and enter their gaseous state.
When current first enters the hair dryer, it goes through the heating element. This part of the machinery is essentially an uncovered, coiled wire. As current runs through it, the coil gets hot very quickly and heats up the air inside the hair dryer that's around it.
The other major component of the machine is the fan. The circular movement of the fan sucks air in through a grate in the side of the hair dryer. Once the air enters the cylinder, it encounters the heating element, rapidly heats up, and is pushed by the moving fan down the barrel of the hair dryer.
Hair dryers have high and low settings because the amount of air that a dryer sends out is proportional to the speed of the motor. If the motor is turning rapidly, then the air is forced out faster than if the motor is turning slowly.