Hair dryers speed the drying process of hair by passing air, which is usually heated, over the wet hair shaft, accelerating the rate of water evaporation.
Excess heat may overly dry the hair, increasing the incidence of splitting or other damage. Hair dryer diffusers can be used to diffuse the stream of air flow so it is weaker but covers a larger area of the hair.
Hair dryers can also be used as a tool to sculpt the hair to a very slight degree. Repeated blow drying can slowly train hair follicles towards the desired direction. Proper technique involves aiming the dryer such that the air does not blow onto the face or scalp, which can cause burns.