The function of breasts is to produce milk for the baby. Each breast has milk producing glands (also called lobules or alveoli) that make milk from the nutrients and water they take from the bloodstream.
Secondly, there are milk ducts that carry the milk to the nipple from the milk glands. The system of milk glands and ducts resembles several bunches of grapes: the glands are the grapes, the ducts are the stems. Like grapes come in bunches, the milk glands and ducts also are organized into several clusters that are called lobes. The breast actually has 15-20 of these lobes or grape bunches in it. From outside, the lobes feel like little nodes or lumps, especially before menstruation.
The space in between the lobes is filled with connective and fatty tissue. Fat also surrounds the whole system of milk ducts and glands. Usually the breasts of young women are mainly glandular tissue, and that is why their breasts are firmer. The softer the breasts are, the more fat they contain.
Note that the breast does not have any muscles (except for tiny ones in the nipples), so no amount of exercise will change their appearance. The breast is supported by semi-elastic bands of tissue called Cooper's ligaments. These ligaments (along with the skin) stretch over time when the gravity pulls the breast down, and that is why the breast will start drooping or sagging.
The breast size and shape in different women varies a lot. Some women have more glandular tissue in their breasts, some have less. Some have more fatty tissue than others. Some have more connective tissue so their breasts are firmer, and yet some women are totally flat-chested.
The size and shape also varies over time in the same woman because of the changes during menstrual cycle, pregnancy, after weaning, and during menopause. Most of the size differences between women are due to the amount of fatty tissue in the breast. But practically all breasts can make milk and help nurture the baby — and that is what makes breasts beautiful!
The rare exception to this is the hypoplastic or underdeveloped breast that does not have much glandular tissue (milk-making glands). Hypoplastic breasts often are small, elongated or tubular shaped, narrow at the chest wall with wide space in between, and often have big areola. Women with these tubular under-developed breasts may have low milk supply, but many of them can with the proper measures develop full supply and breastfeed succesfully.
MOST women's breasts are not totally symmetrical (don't judge by the models and actresses in the media since they have had theirs fixed, pushed up, padded and everything else). Usually, one breast is slightly larger or smaller, higher or lower, or shaped differently than the other (and most of the time it is the left breast that is lower). It is kind of a similar situation to testes in men: usually the left one hangs lower than the right one.
The darker part surrounding the nipple is called the areola. Areolas usually grow in size and get darker during pregnancy — as if making it easier for the baby to spot the place of nourishment. The little bumps on the areola are called Montgomery glands, and they produce oil that lubricates the nipple/areola complex.
The nipple has several tiny openings in it through which the milk flows during lactation. Nipples can sometimes be flat, with a 'line' through them, or inverted, where the nipple is indented inward. Neither situation is serious or dangerous since the baby can usually pull the nipple out. By the way, a teenager's nipple does not necessarily protrude, but it is only truly a flat nipple if it does NOT get erect when stimulated or when cold.
Nipples also come in all different kinds of looks. Some women's nipples are constantly erect whereas some have nipples that only become erect when stimulated by cold or touch. It is common to have some hair on the breast. The nipple can be flat, round, or cylindrical in shape. The areola can be a very narrow ring, or may cover half of a small breast. The color varies from pink to black. Even so, these characteristics don't affect the breastfeeding process — with the exception that sometimes women with flat or inverted nipples have to use special measures to get the baby to feed. You can see pictures of nipples on our nipple gallery page.