Breasts are amazing structures. They're so unique in the animal kingdom that their presence defines an entire group of organisms -- the word "mammal" comes from "mammary," which itself comes from "mamma," the Latin word for breast, udder or teat. Biologists would classify breasts as exocrine glands, or structures that secrete their products through ducts to the external environment. This isn't the same as endocrine glands, which secrete their products directly into the bloodstream.
The product made from breasts, of course, is milk. Milk arrives to the outside world through the nipple, but it begins its life deeper in the breast, in clusters of cells known as alveoli. These clusters form lobules, which themselves create larger structures known as lobes. As the alveoli produce milk, the fluid passes through thin tubes -- lactiferous ducts -- that lead to openings in the nipple. Fibrous tissue and fat fill the spaces between the lobules and the ducts, and the whole structure sits on top of the pectoralis muscles of the chest. A network of lymph vessels and nodes surrounds all of this tissue and extends upward into the armpit.
In many women, this tissue functions properly and never causes problems. Next, though, we'll look at what happens when it does.