There are many types of hair-loss, also called alopecia:
Involutional alopecia is a natural condition in which the hair gradually thins with age. More hair follicles go into the resting phase, and the remaining hairs become shorter and fewer in number.
Androgenic alopecia is a genetic condition that can affect both men and women. Men with this condition, called male pattern baldness, can begin suffering hair-loss as early as their teens or early 20s. It's characterized by a receding hairline and gradual disappearance of hair from the crown and frontal scalp. Women with this condition, called female pattern baldness, don't experience noticeable thinning until their 40s or later. Women experience a general thinning over the entire scalp, with the most extensive hair loss at the crown.
Alopecia areata often starts suddenly and causes patchy hair loss in children and young adults. This condition may result in complete baldness (alopecia totalis). But in about 90% of people with the condition, the hair returns within a few years.
Alopecia universalis causes all body hair to fall out, including the eyebrows, eyelashes, and pubic hair.
Trichotillomania, seen most frequently in children, is a psychological disorder in which a person pulls out one's own hair.
Telogen effluvium is temporary hair thinning over the scalp that occurs because of changes in the growth cycle of hair. A large number of hairs enter the resting phase at the same time, causing hair shedding and subsequent thinning.
Scarring alopecias result in permanent loss of hair. Inflammatory skin conditions (cellulitis, folliculitis, acne), and other skin disorders (such as some forms of lupus and lichen planus) often result in scars that destroy the ability of the hair to regenerate. Hot combs and hair too tightly woven and pulled can also result in permanent hair loss.