A common misconception is that women have breasts so that they can feed babies by producing milk. However, the truth is that the mammary glands that secrete the milk in the breasts makes up a pretty small percentage of the overall breast tissue. Most of the average human female breast is actually adipose tissue (fat) and connective tissue. Women with small breasts can nurse a baby just as well as women with large breasts...
The real evolutionary purpose of women having breasts is to attract the male of the species.
Anatomically, the breasts sit over the pectoralis major muscle and usually extend from the level of the 2nd rib to the level of the 6th rib anteriorly. The superior lateral quadrant of the breast extends diagonally upwards in an 'axillary tail'. A thin layer of mammary tissue extends from the clavicle above to the seventh or eight ribs below and from the midline to the edge of the latissimus dorsi posteriorly.
Important parts of the breasts: mammary glands, the axillary tail (tumours most likely to occur here), the lobules, Cooper's ligaments, the areola and the nipple.