Breast

"The breast is the upper ventral region of the torso of a primate, in left and right sides, which in a female contains the mammary gland that secretes milk used to feed infants. Both men and women develop breasts from the same embryological tissues. However, at puberty, female sex hormones, mainly estrogen, promote breast development, which does not occur in men, due to the higher amount of testosterone. As a result, women's breasts become far more prominent than those of men.
During pregnancy, the breast is responsive to a complex interplay of hormones that cause tissue development and enlargement in order to produce milk. Three such hormones are estrogen, progesterone and prolactin, which cause glandular tissue in the breast and the uterus to change during the menstrual cycle. Each breast contains 15–20 lobes. The subcutaneous adipose tissue covering the lobes gives the breast its size and shape. Each lobe is composed of many lobules, at the end of which are sacs where milk is produced in response to hormonal signals."

Galactorrhea

Galactorrhea is a milky nipple discharge unrelated to the normal milk production of breast-feeding. Galactorrhea itself isn't a disease, but it's a sign of an underlying problem. Although it occurs most often in women, galactorrhea can happen in men and even sometimes in infants.
Excessive breast stimulation, medication side effects, or disorders of the pituitary gland all may contribute to galactorrhea. Often, galactorrhea results from increased levels of prolactin, the hormone that stimulates milk production. Sometimes, the cause of galactorrhea can't be determined, and the condition goes away on its own.