Estrogen is a sex hormone that is responsible for female sex characteristics and helps regulate ovulation and pregnancy. It is produced by the ovaries and, in smaller quantities, by the testicles.
During puberty, estrogen is responsible for the development of secondary sexual characteristics in females, such as the development of the breasts and hips. It is also responsible for females having finer body hair and smoother skin than men generally do. It is also vital in ovulation and fetal development.
Too much estrogen in males can result in development of female sexual characteristics (particularly enlarged breasts) and decreased sexual desire.