Breast tissue is made up of fat tissue and epithelial cells. The fat layers cover an area spanning from the underarm to the collar bone and breastbone. The epithelial cells form 12 to 20 sections called lobes, which in turn form smaller lobules which produce milk after childbirth. Within the fat tissue, there is a network of fibrous connective tissue that contains the blood vessels, lymph vessels, lymph nodes, and nerves.
Most breast cancers arise out of the lobules and ducts (conduits for the milk to exit through the nipple), and are therefore described as lobular or ductal carcinomas.
Information taken from Memorial Sloan Kettering Cancer Center.