Medically, hair loss is known as androgenetic alopecia, or male/female pattern hair loss, brought on by the hormone Dihydrotestosterone (DHT). When DHT levels rise, hair follicles get damaged - thus stopping hair growth. Men and women generally have different responses to DHT.
The first signs of male pattern baldness are a receding hairline which eventually forms a characteristic "M" shape and thinning hair around the crown area, often progressing to baldness. A woman's hairline rarely recedes and rarely becomes bald, but alopecia can begin with a gradual thinning at the parting line.