Hair is a protein filament that grows through the epidermis from follicles deep within the dermis. The fine, soft hair found on many nonhuman mammals is typically called fur; wool is the characteristically curly hair found on sheep and goats. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class. Although other non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" in the scientific sense. So-called "hairs" (trichomes) are also found on plants. The projections on arthropods such as insects and spiders are actually insect bristles, composed of a polysaccharide called chitin. There are varieties of cats, dogs, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life. The main component of hair fiber is keratin.

The hair can be divided into three parts length-wise, (1) the bulb, a swelling at the base which originates from the dermis, (2) the root, which is the hair lying beneath the skin surface, and (3) the shaft, which is the hair above the skin surface. In cross-section, there are also three parts, (1) the medulla, an area in the core which contains loose cells and airspaces (2) the cortex, which contains densely packed keratin and (3) the cuticle, which is a single layer of cells arranged like roof shingles.

Hair types

Human beings have three distinct types of hair:

• Lanugo is fine hair that covers nearly the entire body of a fetus. Unless born prematurely, the fetus loses this layer of hair before birth. Lanugo also sometimes returns in cases of malnutrition or extreme anorexia nervosa, as the starved body attempts to insulate itself.
• Vellus hair is extremely short, fine, scarcely noticeable hair that covers most of the human body in both sexes.
• Terminal hair is fully developed hair, which is generally longer, coarser, thicker, and darker than vellus hair.

Balding and greying

The tendency of older people to develop grey hair is due to "a massive build up of hydrogen peroxide due to wear and tear of our hair follicles ...1 winds up blocking the normal synthesis of melanin, our hair's natural pigment." Grey hair is considered to be a characteristic of normal aging. The age at which this occurs varies from person to person, but in general nearly everyone 75 years or older has grey hair, and in general men tend to become grey at younger ages than women.

It should be noted however, that grey hair in itself is not actually grey; the grey head of hair is a result of a combination of the dark and white/colourless hair forming an overall 'grey' appearance to the observer. As such, people starting out with very pale blond hair usually develop white hair instead of grey hair when aging. Red hair usually doesn't turn grey with age; rather it becomes a sandy colour and afterward turns white. Some degree of scalp hair loss or thinning generally accompanies aging in both males and females, and it's estimated that half of all men are affected by male pattern baldness by the time they are 50"Uncovering the bald truth about hair losser." Springfield News-leader, May 10, 2005. "Half of men" estimate is made by the American Academy of Dermatology and specifically estimates prevalence in the U.S. population, though this should reflect prevalence in other populations.. The tendency toward baldness is a trait shared by a number of other primate species, and is thought to have evolutionary roots.