Male-pattern hair loss (MPHL), also known as androgenic alopecia and male pattern baldness, is hair loss that occurs due to an underlying susceptibility of hair follicles to shrinkage due to the influence of androgenic hormones. Male-pattern hair loss is the most common cause of hair loss and will affect up to 70% of men and 40% of women at some point in their lifetimes. Men typically present with progressive hair loss at the temples and vertex balding, whereas women typically present with diffuse hair loss over the top of their scalps source: https://en.wikipedia.org/wiki/Male-pattern_hair_loss
Hair is made in hair follicles which are like tiny pouches just under the skin surface. A hair normally grows from each follicle for about three years. It is then shed and a new hair grows from the follicle. This cycle of hair growth, shedding and new growth goes on throughout life. The following is thought to occur in men as they gradually become bald:
Affected hair follicles on the scalp gradually become smaller than normal.
As the follicle shrinks, each new hair is thinner than the previous one.
Before falling out, each new hair grows for much less time than the normal three years or so.
Eventually, all that remains is a much smaller hair follicle and a thin stump of hair that does not grow out to the skin surface. source: http://patient.info/health/male-pattern-baldness